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<div class="section" id="module-typing">
<span id="typing-support-for-type-hints"></span><h1>26.1. <a class="reference internal" href="#module-typing" title="typing: Support for type hints (see PEP 484)."><code class="xref py py-mod docutils literal"><span class="pre">typing</span></code></a> — Support for type hints<a class="headerlink" href="#module-typing" title="Permalink to this headline">¶</a></h1>
<div class="versionadded">
<p><span class="versionmodified">New in version 3.5.</span></p>
</div>
<p><strong>Source code:</strong> <a class="reference external" href="https://hg.python.org/cpython/file/3.5/Lib/typing.py">Lib/typing.py</a></p>
<hr class="docutils" />
<p>This module supports type hints as specified by <span class="target" id="index-0"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0484"><strong>PEP 484</strong></a>. The most
fundamental support consists of the types <a class="reference internal" href="#typing.Any" title="typing.Any"><code class="xref py py-data docutils literal"><span class="pre">Any</span></code></a>, <a class="reference internal" href="#typing.Union" title="typing.Union"><code class="xref py py-data docutils literal"><span class="pre">Union</span></code></a>,
<a class="reference internal" href="#typing.Tuple" title="typing.Tuple"><code class="xref py py-data docutils literal"><span class="pre">Tuple</span></code></a>, <a class="reference internal" href="#typing.Callable" title="typing.Callable"><code class="xref py py-data docutils literal"><span class="pre">Callable</span></code></a>, <a class="reference internal" href="#typing.TypeVar" title="typing.TypeVar"><code class="xref py py-class docutils literal"><span class="pre">TypeVar</span></code></a>, and
<a class="reference internal" href="#typing.Generic" title="typing.Generic"><code class="xref py py-class docutils literal"><span class="pre">Generic</span></code></a>. For full specification please see <span class="target" id="index-1"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0484"><strong>PEP 484</strong></a>. For
a simplified introduction to type hints see <span class="target" id="index-2"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0483"><strong>PEP 483</strong></a>.</p>
<p>The function below takes and returns a string and is annotated as follows:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">greeting</span><span class="p">(</span><span class="n">name</span><span class="p">:</span> <span class="nb">str</span><span class="p">)</span> <span class="o">-></span> <span class="nb">str</span><span class="p">:</span>
<span class="k">return</span> <span class="s1">'Hello '</span> <span class="o">+</span> <span class="n">name</span>
</pre></div>
</div>
<p>In the function <code class="docutils literal"><span class="pre">greeting</span></code>, the argument <code class="docutils literal"><span class="pre">name</span></code> is expected to be of type
<a class="reference internal" href="stdtypes.html#str" title="str"><code class="xref py py-class docutils literal"><span class="pre">str</span></code></a> and the return type <a class="reference internal" href="stdtypes.html#str" title="str"><code class="xref py py-class docutils literal"><span class="pre">str</span></code></a>. Subtypes are accepted as
arguments.</p>
<div class="section" id="type-aliases">
<h2>26.1.1. Type aliases<a class="headerlink" href="#type-aliases" title="Permalink to this headline">¶</a></h2>
<p>A type alias is defined by assigning the type to the alias. In this example,
<code class="docutils literal"><span class="pre">Vector</span></code> and <code class="docutils literal"><span class="pre">List[float]</span></code> will be treated as interchangeable synonyms:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">typing</span> <span class="k">import</span> <span class="n">List</span>
<span class="n">Vector</span> <span class="o">=</span> <span class="n">List</span><span class="p">[</span><span class="nb">float</span><span class="p">]</span>
<span class="k">def</span> <span class="nf">scale</span><span class="p">(</span><span class="n">scalar</span><span class="p">:</span> <span class="nb">float</span><span class="p">,</span> <span class="n">vector</span><span class="p">:</span> <span class="n">Vector</span><span class="p">)</span> <span class="o">-></span> <span class="n">Vector</span><span class="p">:</span>
<span class="k">return</span> <span class="p">[</span><span class="n">scalar</span> <span class="o">*</span> <span class="n">num</span> <span class="k">for</span> <span class="n">num</span> <span class="ow">in</span> <span class="n">vector</span><span class="p">]</span>
<span class="c1"># typechecks; a list of floats qualifies as a Vector.</span>
<span class="n">new_vector</span> <span class="o">=</span> <span class="n">scale</span><span class="p">(</span><span class="mf">2.0</span><span class="p">,</span> <span class="p">[</span><span class="mf">1.0</span><span class="p">,</span> <span class="o">-</span><span class="mf">4.2</span><span class="p">,</span> <span class="mf">5.4</span><span class="p">])</span>
</pre></div>
</div>
<p>Type aliases are useful for simplifying complex type signatures. For example:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">typing</span> <span class="k">import</span> <span class="n">Dict</span><span class="p">,</span> <span class="n">Tuple</span><span class="p">,</span> <span class="n">List</span>
<span class="n">ConnectionOptions</span> <span class="o">=</span> <span class="n">Dict</span><span class="p">[</span><span class="nb">str</span><span class="p">,</span> <span class="nb">str</span><span class="p">]</span>
<span class="n">Address</span> <span class="o">=</span> <span class="n">Tuple</span><span class="p">[</span><span class="nb">str</span><span class="p">,</span> <span class="nb">int</span><span class="p">]</span>
<span class="n">Server</span> <span class="o">=</span> <span class="n">Tuple</span><span class="p">[</span><span class="n">Address</span><span class="p">,</span> <span class="n">ConnectionOptions</span><span class="p">]</span>
<span class="k">def</span> <span class="nf">broadcast_message</span><span class="p">(</span><span class="n">message</span><span class="p">:</span> <span class="nb">str</span><span class="p">,</span> <span class="n">servers</span><span class="p">:</span> <span class="n">List</span><span class="p">[</span><span class="n">Server</span><span class="p">])</span> <span class="o">-></span> <span class="kc">None</span><span class="p">:</span>
<span class="o">...</span>
<span class="c1"># The static type checker will treat the previous type signature as</span>
<span class="c1"># being exactly equivalent to this one.</span>
<span class="k">def</span> <span class="nf">broadcast_message</span><span class="p">(</span>
<span class="n">message</span><span class="p">:</span> <span class="nb">str</span><span class="p">,</span>
<span class="n">servers</span><span class="p">:</span> <span class="n">List</span><span class="p">[</span><span class="n">Tuple</span><span class="p">[</span><span class="n">Tuple</span><span class="p">[</span><span class="nb">str</span><span class="p">,</span> <span class="nb">int</span><span class="p">],</span> <span class="n">Dict</span><span class="p">[</span><span class="nb">str</span><span class="p">,</span> <span class="nb">str</span><span class="p">]]])</span> <span class="o">-></span> <span class="kc">None</span><span class="p">:</span>
<span class="o">...</span>
</pre></div>
</div>
<p>Note that <code class="docutils literal"><span class="pre">None</span></code> as a type hint is a special case and is replaced by
<code class="docutils literal"><span class="pre">type(None)</span></code>.</p>
</div>
<div class="section" id="newtype">
<span id="distinct"></span><h2>26.1.2. NewType<a class="headerlink" href="#newtype" title="Permalink to this headline">¶</a></h2>
<p>Use the <a class="reference internal" href="#typing.NewType" title="typing.NewType"><code class="xref py py-func docutils literal"><span class="pre">NewType()</span></code></a> helper function to create distinct types:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">typing</span> <span class="k">import</span> <span class="n">NewType</span>
<span class="n">UserId</span> <span class="o">=</span> <span class="n">NewType</span><span class="p">(</span><span class="s1">'UserId'</span><span class="p">,</span> <span class="nb">int</span><span class="p">)</span>
<span class="n">some_id</span> <span class="o">=</span> <span class="n">UserId</span><span class="p">(</span><span class="mi">524313</span><span class="p">)</span>
</pre></div>
</div>
<p>The static type checker will treat the new type as if it were a subclass
of the original type. This is useful in helping catch logical errors:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">get_user_name</span><span class="p">(</span><span class="n">user_id</span><span class="p">:</span> <span class="n">UserId</span><span class="p">)</span> <span class="o">-></span> <span class="nb">str</span><span class="p">:</span>
<span class="o">...</span>
<span class="c1"># typechecks</span>
<span class="n">user_a</span> <span class="o">=</span> <span class="n">get_user_name</span><span class="p">(</span><span class="n">UserId</span><span class="p">(</span><span class="mi">42351</span><span class="p">))</span>
<span class="c1"># does not typecheck; an int is not a UserId</span>
<span class="n">user_b</span> <span class="o">=</span> <span class="n">get_user_name</span><span class="p">(</span><span class="o">-</span><span class="mi">1</span><span class="p">)</span>
</pre></div>
</div>
<p>You may still perform all <code class="docutils literal"><span class="pre">int</span></code> operations on a variable of type <code class="docutils literal"><span class="pre">UserId</span></code>,
but the result will always be of type <code class="docutils literal"><span class="pre">int</span></code>. This lets you pass in a
<code class="docutils literal"><span class="pre">UserId</span></code> wherever an <code class="docutils literal"><span class="pre">int</span></code> might be expected, but will prevent you from
accidentally creating a <code class="docutils literal"><span class="pre">UserId</span></code> in an invalid way:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="c1"># 'output' is of type 'int', not 'UserId'</span>
<span class="n">output</span> <span class="o">=</span> <span class="n">UserId</span><span class="p">(</span><span class="mi">23413</span><span class="p">)</span> <span class="o">+</span> <span class="n">UserId</span><span class="p">(</span><span class="mi">54341</span><span class="p">)</span>
</pre></div>
</div>
<p>Note that these checks are enforced only by the static type checker. At runtime
the statement <code class="docutils literal"><span class="pre">Derived</span> <span class="pre">=</span> <span class="pre">NewType('Derived',</span> <span class="pre">Base)</span></code> will make <code class="docutils literal"><span class="pre">Derived</span></code> a
function that immediately returns whatever parameter you pass it. That means
the expression <code class="docutils literal"><span class="pre">Derived(some_value)</span></code> does not create a new class or introduce
any overhead beyond that of a regular function call.</p>
<p>More precisely, the expression <code class="docutils literal"><span class="pre">some_value</span> <span class="pre">is</span> <span class="pre">Derived(some_value)</span></code> is always
true at runtime.</p>
<p>This also means that it is not possible to create a subtype of <code class="docutils literal"><span class="pre">Derived</span></code>
since it is an identity function at runtime, not an actual type. Similarly, it
is not possible to create another <a class="reference internal" href="#typing.NewType" title="typing.NewType"><code class="xref py py-func docutils literal"><span class="pre">NewType()</span></code></a> based on a <code class="docutils literal"><span class="pre">Derived</span></code> type:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">typing</span> <span class="k">import</span> <span class="n">NewType</span>
<span class="n">UserId</span> <span class="o">=</span> <span class="n">NewType</span><span class="p">(</span><span class="s1">'UserId'</span><span class="p">,</span> <span class="nb">int</span><span class="p">)</span>
<span class="c1"># Fails at runtime and does not typecheck</span>
<span class="k">class</span> <span class="nc">AdminUserId</span><span class="p">(</span><span class="n">UserId</span><span class="p">):</span> <span class="k">pass</span>
<span class="c1"># Also does not typecheck</span>
<span class="n">ProUserId</span> <span class="o">=</span> <span class="n">NewType</span><span class="p">(</span><span class="s1">'ProUserId'</span><span class="p">,</span> <span class="n">UserId</span><span class="p">)</span>
</pre></div>
</div>
<p>See <span class="target" id="index-3"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0484"><strong>PEP 484</strong></a> for more details.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p>Recall that the use of a type alias declares two types to be <em>equivalent</em> to
one another. Doing <code class="docutils literal"><span class="pre">Alias</span> <span class="pre">=</span> <span class="pre">Original</span></code> will make the static type checker
treat <code class="docutils literal"><span class="pre">Alias</span></code> as being <em>exactly equivalent</em> to <code class="docutils literal"><span class="pre">Original</span></code> in all cases.
This is useful when you want to simplify complex type signatures.</p>
<p class="last">In contrast, <code class="docutils literal"><span class="pre">NewType</span></code> declares one type to be a <em>subtype</em> of another.
Doing <code class="docutils literal"><span class="pre">Derived</span> <span class="pre">=</span> <span class="pre">NewType('Derived',</span> <span class="pre">Original)</span></code> will make the static type
checker treat <code class="docutils literal"><span class="pre">Derived</span></code> as a <em>subclass</em> of <code class="docutils literal"><span class="pre">Original</span></code>, which means a
value of type <code class="docutils literal"><span class="pre">Original</span></code> cannot be used in places where a value of type
<code class="docutils literal"><span class="pre">Derived</span></code> is expected. This is useful when you want to prevent logic
errors with minimal runtime cost.</p>
</div>
</div>
<div class="section" id="callable">
<h2>26.1.3. Callable<a class="headerlink" href="#callable" title="Permalink to this headline">¶</a></h2>
<p>Frameworks expecting callback functions of specific signatures might be
type hinted using <code class="docutils literal"><span class="pre">Callable[[Arg1Type,</span> <span class="pre">Arg2Type],</span> <span class="pre">ReturnType]</span></code>.</p>
<p>For example:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">typing</span> <span class="k">import</span> <span class="n">Callable</span>
<span class="k">def</span> <span class="nf">feeder</span><span class="p">(</span><span class="n">get_next_item</span><span class="p">:</span> <span class="n">Callable</span><span class="p">[[],</span> <span class="nb">str</span><span class="p">])</span> <span class="o">-></span> <span class="kc">None</span><span class="p">:</span>
<span class="c1"># Body</span>
<span class="k">def</span> <span class="nf">async_query</span><span class="p">(</span><span class="n">on_success</span><span class="p">:</span> <span class="n">Callable</span><span class="p">[[</span><span class="nb">int</span><span class="p">],</span> <span class="kc">None</span><span class="p">],</span>
<span class="n">on_error</span><span class="p">:</span> <span class="n">Callable</span><span class="p">[[</span><span class="nb">int</span><span class="p">,</span> <span class="ne">Exception</span><span class="p">],</span> <span class="kc">None</span><span class="p">])</span> <span class="o">-></span> <span class="kc">None</span><span class="p">:</span>
<span class="c1"># Body</span>
</pre></div>
</div>
<p>It is possible to declare the return type of a callable without specifying
the call signature by substituting a literal ellipsis
for the list of arguments in the type hint: <code class="docutils literal"><span class="pre">Callable[...,</span> <span class="pre">ReturnType]</span></code>.</p>
</div>
<div class="section" id="generics">
<h2>26.1.4. Generics<a class="headerlink" href="#generics" title="Permalink to this headline">¶</a></h2>
<p>Since type information about objects kept in containers cannot be statically
inferred in a generic way, abstract base classes have been extended to support
subscription to denote expected types for container elements.</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">typing</span> <span class="k">import</span> <span class="n">Mapping</span><span class="p">,</span> <span class="n">Sequence</span>
<span class="k">def</span> <span class="nf">notify_by_email</span><span class="p">(</span><span class="n">employees</span><span class="p">:</span> <span class="n">Sequence</span><span class="p">[</span><span class="n">Employee</span><span class="p">],</span>
<span class="n">overrides</span><span class="p">:</span> <span class="n">Mapping</span><span class="p">[</span><span class="nb">str</span><span class="p">,</span> <span class="nb">str</span><span class="p">])</span> <span class="o">-></span> <span class="kc">None</span><span class="p">:</span> <span class="o">...</span>
</pre></div>
</div>
<p>Generics can be parametrized by using a new factory available in typing
called <a class="reference internal" href="#typing.TypeVar" title="typing.TypeVar"><code class="xref py py-class docutils literal"><span class="pre">TypeVar</span></code></a>.</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">typing</span> <span class="k">import</span> <span class="n">Sequence</span><span class="p">,</span> <span class="n">TypeVar</span>
<span class="n">T</span> <span class="o">=</span> <span class="n">TypeVar</span><span class="p">(</span><span class="s1">'T'</span><span class="p">)</span> <span class="c1"># Declare type variable</span>
<span class="k">def</span> <span class="nf">first</span><span class="p">(</span><span class="n">l</span><span class="p">:</span> <span class="n">Sequence</span><span class="p">[</span><span class="n">T</span><span class="p">])</span> <span class="o">-></span> <span class="n">T</span><span class="p">:</span> <span class="c1"># Generic function</span>
<span class="k">return</span> <span class="n">l</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span>
</pre></div>
</div>
</div>
<div class="section" id="user-defined-generic-types">
<h2>26.1.5. User-defined generic types<a class="headerlink" href="#user-defined-generic-types" title="Permalink to this headline">¶</a></h2>
<p>A user-defined class can be defined as a generic class.</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">typing</span> <span class="k">import</span> <span class="n">TypeVar</span><span class="p">,</span> <span class="n">Generic</span>
<span class="kn">from</span> <span class="nn">logging</span> <span class="k">import</span> <span class="n">Logger</span>
<span class="n">T</span> <span class="o">=</span> <span class="n">TypeVar</span><span class="p">(</span><span class="s1">'T'</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">LoggedVar</span><span class="p">(</span><span class="n">Generic</span><span class="p">[</span><span class="n">T</span><span class="p">]):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">value</span><span class="p">:</span> <span class="n">T</span><span class="p">,</span> <span class="n">name</span><span class="p">:</span> <span class="nb">str</span><span class="p">,</span> <span class="n">logger</span><span class="p">:</span> <span class="n">Logger</span><span class="p">)</span> <span class="o">-></span> <span class="kc">None</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="n">name</span>
<span class="bp">self</span><span class="o">.</span><span class="n">logger</span> <span class="o">=</span> <span class="n">logger</span>
<span class="bp">self</span><span class="o">.</span><span class="n">value</span> <span class="o">=</span> <span class="n">value</span>
<span class="k">def</span> <span class="nf">set</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">new</span><span class="p">:</span> <span class="n">T</span><span class="p">)</span> <span class="o">-></span> <span class="kc">None</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="s1">'Set '</span> <span class="o">+</span> <span class="nb">repr</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">value</span><span class="p">))</span>
<span class="bp">self</span><span class="o">.</span><span class="n">value</span> <span class="o">=</span> <span class="n">new</span>
<span class="k">def</span> <span class="nf">get</span><span class="p">(</span><span class="bp">self</span><span class="p">)</span> <span class="o">-></span> <span class="n">T</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="s1">'Get '</span> <span class="o">+</span> <span class="nb">repr</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">value</span><span class="p">))</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">value</span>
<span class="k">def</span> <span class="nf">log</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">message</span><span class="p">:</span> <span class="nb">str</span><span class="p">)</span> <span class="o">-></span> <span class="kc">None</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">logger</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">'</span><span class="si">%s</span><span class="s1">: </span><span class="si">%s</span><span class="s1">'</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">name</span><span class="p">,</span> <span class="n">message</span><span class="p">)</span>
</pre></div>
</div>
<p><code class="docutils literal"><span class="pre">Generic[T]</span></code> as a base class defines that the class <code class="docutils literal"><span class="pre">LoggedVar</span></code> takes a
single type parameter <code class="docutils literal"><span class="pre">T</span></code> . This also makes <code class="docutils literal"><span class="pre">T</span></code> valid as a type within the
class body.</p>
<p>The <a class="reference internal" href="#typing.Generic" title="typing.Generic"><code class="xref py py-class docutils literal"><span class="pre">Generic</span></code></a> base class uses a metaclass that defines
<a class="reference internal" href="../reference/datamodel.html#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal"><span class="pre">__getitem__()</span></code></a> so that <code class="docutils literal"><span class="pre">LoggedVar[t]</span></code> is valid as a type:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">typing</span> <span class="k">import</span> <span class="n">Iterable</span>
<span class="k">def</span> <span class="nf">zero_all_vars</span><span class="p">(</span><span class="nb">vars</span><span class="p">:</span> <span class="n">Iterable</span><span class="p">[</span><span class="n">LoggedVar</span><span class="p">[</span><span class="nb">int</span><span class="p">]])</span> <span class="o">-></span> <span class="kc">None</span><span class="p">:</span>
<span class="k">for</span> <span class="n">var</span> <span class="ow">in</span> <span class="nb">vars</span><span class="p">:</span>
<span class="n">var</span><span class="o">.</span><span class="n">set</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
</pre></div>
</div>
<p>A generic type can have any number of type variables, and type variables may
be constrained:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">typing</span> <span class="k">import</span> <span class="n">TypeVar</span><span class="p">,</span> <span class="n">Generic</span>
<span class="o">...</span>
<span class="n">T</span> <span class="o">=</span> <span class="n">TypeVar</span><span class="p">(</span><span class="s1">'T'</span><span class="p">)</span>
<span class="n">S</span> <span class="o">=</span> <span class="n">TypeVar</span><span class="p">(</span><span class="s1">'S'</span><span class="p">,</span> <span class="nb">int</span><span class="p">,</span> <span class="nb">str</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">StrangePair</span><span class="p">(</span><span class="n">Generic</span><span class="p">[</span><span class="n">T</span><span class="p">,</span> <span class="n">S</span><span class="p">]):</span>
<span class="o">...</span>
</pre></div>
</div>
<p>Each type variable argument to <a class="reference internal" href="#typing.Generic" title="typing.Generic"><code class="xref py py-class docutils literal"><span class="pre">Generic</span></code></a> must be distinct.
This is thus invalid:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">typing</span> <span class="k">import</span> <span class="n">TypeVar</span><span class="p">,</span> <span class="n">Generic</span>
<span class="o">...</span>
<span class="n">T</span> <span class="o">=</span> <span class="n">TypeVar</span><span class="p">(</span><span class="s1">'T'</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">Pair</span><span class="p">(</span><span class="n">Generic</span><span class="p">[</span><span class="n">T</span><span class="p">,</span> <span class="n">T</span><span class="p">]):</span> <span class="c1"># INVALID</span>
<span class="o">...</span>
</pre></div>
</div>
<p>You can use multiple inheritance with <a class="reference internal" href="#typing.Generic" title="typing.Generic"><code class="xref py py-class docutils literal"><span class="pre">Generic</span></code></a>:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">typing</span> <span class="k">import</span> <span class="n">TypeVar</span><span class="p">,</span> <span class="n">Generic</span><span class="p">,</span> <span class="n">Sized</span>
<span class="n">T</span> <span class="o">=</span> <span class="n">TypeVar</span><span class="p">(</span><span class="s1">'T'</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">LinkedList</span><span class="p">(</span><span class="n">Sized</span><span class="p">,</span> <span class="n">Generic</span><span class="p">[</span><span class="n">T</span><span class="p">]):</span>
<span class="o">...</span>
</pre></div>
</div>
<p>When inheriting from generic classes, some type variables could be fixed:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">typing</span> <span class="k">import</span> <span class="n">TypeVar</span><span class="p">,</span> <span class="n">Mapping</span>
<span class="n">T</span> <span class="o">=</span> <span class="n">TypeVar</span><span class="p">(</span><span class="s1">'T'</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">MyDict</span><span class="p">(</span><span class="n">Mapping</span><span class="p">[</span><span class="nb">str</span><span class="p">,</span> <span class="n">T</span><span class="p">]):</span>
<span class="o">...</span>
</pre></div>
</div>
<p>In this case <code class="docutils literal"><span class="pre">MyDict</span></code> has a single parameter, <code class="docutils literal"><span class="pre">T</span></code>.</p>
<p>Using a generic class without specifying type parameters assumes
<a class="reference internal" href="#typing.Any" title="typing.Any"><code class="xref py py-data docutils literal"><span class="pre">Any</span></code></a> for each position. In the following example, <code class="docutils literal"><span class="pre">MyIterable</span></code> is
not generic but implicitly inherits from <code class="docutils literal"><span class="pre">Iterable[Any]</span></code>:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">typing</span> <span class="k">import</span> <span class="n">Iterable</span>
<span class="k">class</span> <span class="nc">MyIterable</span><span class="p">(</span><span class="n">Iterable</span><span class="p">):</span> <span class="c1"># Same as Iterable[Any]</span>
</pre></div>
</div>
<p>User defined generic type aliases are also supported. Examples:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">typing</span> <span class="k">import</span> <span class="n">TypeVar</span><span class="p">,</span> <span class="n">Iterable</span><span class="p">,</span> <span class="n">Tuple</span><span class="p">,</span> <span class="n">Union</span>
<span class="n">S</span> <span class="o">=</span> <span class="n">TypeVar</span><span class="p">(</span><span class="s1">'S'</span><span class="p">)</span>
<span class="n">Response</span> <span class="o">=</span> <span class="n">Union</span><span class="p">[</span><span class="n">Iterable</span><span class="p">[</span><span class="n">S</span><span class="p">],</span> <span class="nb">int</span><span class="p">]</span>
<span class="c1"># Return type here is same as Union[Iterable[str], int]</span>
<span class="k">def</span> <span class="nf">response</span><span class="p">(</span><span class="n">query</span><span class="p">:</span> <span class="nb">str</span><span class="p">)</span> <span class="o">-></span> <span class="n">Response</span><span class="p">[</span><span class="nb">str</span><span class="p">]:</span>
<span class="o">...</span>
<span class="n">T</span> <span class="o">=</span> <span class="n">TypeVar</span><span class="p">(</span><span class="s1">'T'</span><span class="p">,</span> <span class="nb">int</span><span class="p">,</span> <span class="nb">float</span><span class="p">,</span> <span class="nb">complex</span><span class="p">)</span>
<span class="n">Vec</span> <span class="o">=</span> <span class="n">Iterable</span><span class="p">[</span><span class="n">Tuple</span><span class="p">[</span><span class="n">T</span><span class="p">,</span> <span class="n">T</span><span class="p">]]</span>
<span class="k">def</span> <span class="nf">inproduct</span><span class="p">(</span><span class="n">v</span><span class="p">:</span> <span class="n">Vec</span><span class="p">[</span><span class="n">T</span><span class="p">])</span> <span class="o">-></span> <span class="n">T</span><span class="p">:</span> <span class="c1"># Same as Iterable[Tuple[T, T]]</span>
<span class="k">return</span> <span class="nb">sum</span><span class="p">(</span><span class="n">x</span><span class="o">*</span><span class="n">y</span> <span class="k">for</span> <span class="n">x</span><span class="p">,</span> <span class="n">y</span> <span class="ow">in</span> <span class="n">v</span><span class="p">)</span>
</pre></div>
</div>
<p>The metaclass used by <a class="reference internal" href="#typing.Generic" title="typing.Generic"><code class="xref py py-class docutils literal"><span class="pre">Generic</span></code></a> is a subclass of <a class="reference internal" href="abc.html#abc.ABCMeta" title="abc.ABCMeta"><code class="xref py py-class docutils literal"><span class="pre">abc.ABCMeta</span></code></a>.
A generic class can be an ABC by including abstract methods or properties,
and generic classes can also have ABCs as base classes without a metaclass
conflict. Generic metaclasses are not supported. The outcome of parameterizing
generics is cached, and most types in the typing module are hashable and
comparable for equality.</p>
</div>
<div class="section" id="the-any-type">
<h2>26.1.6. The <a class="reference internal" href="#typing.Any" title="typing.Any"><code class="xref py py-data docutils literal"><span class="pre">Any</span></code></a> type<a class="headerlink" href="#the-any-type" title="Permalink to this headline">¶</a></h2>
<p>A special kind of type is <a class="reference internal" href="#typing.Any" title="typing.Any"><code class="xref py py-data docutils literal"><span class="pre">Any</span></code></a>. A static type checker will treat
every type as being compatible with <a class="reference internal" href="#typing.Any" title="typing.Any"><code class="xref py py-data docutils literal"><span class="pre">Any</span></code></a> and <a class="reference internal" href="#typing.Any" title="typing.Any"><code class="xref py py-data docutils literal"><span class="pre">Any</span></code></a> as being
compatible with every type.</p>
<p>This means that it is possible to perform any operation or method call on a
value of type on <a class="reference internal" href="#typing.Any" title="typing.Any"><code class="xref py py-data docutils literal"><span class="pre">Any</span></code></a> and assign it to any variable:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">typing</span> <span class="k">import</span> <span class="n">Any</span>
<span class="n">a</span> <span class="o">=</span> <span class="kc">None</span> <span class="c1"># type: Any</span>
<span class="n">a</span> <span class="o">=</span> <span class="p">[]</span> <span class="c1"># OK</span>
<span class="n">a</span> <span class="o">=</span> <span class="mi">2</span> <span class="c1"># OK</span>
<span class="n">s</span> <span class="o">=</span> <span class="s1">''</span> <span class="c1"># type: str</span>
<span class="n">s</span> <span class="o">=</span> <span class="n">a</span> <span class="c1"># OK</span>
<span class="k">def</span> <span class="nf">foo</span><span class="p">(</span><span class="n">item</span><span class="p">:</span> <span class="n">Any</span><span class="p">)</span> <span class="o">-></span> <span class="nb">int</span><span class="p">:</span>
<span class="c1"># Typechecks; 'item' could be any type,</span>
<span class="c1"># and that type might have a 'bar' method</span>
<span class="n">item</span><span class="o">.</span><span class="n">bar</span><span class="p">()</span>
<span class="o">...</span>
</pre></div>
</div>
<p>Notice that no typechecking is performed when assigning a value of type
<a class="reference internal" href="#typing.Any" title="typing.Any"><code class="xref py py-data docutils literal"><span class="pre">Any</span></code></a> to a more precise type. For example, the static type checker did
not report an error when assigning <code class="docutils literal"><span class="pre">a</span></code> to <code class="docutils literal"><span class="pre">s</span></code> even though <code class="docutils literal"><span class="pre">s</span></code> was
declared to be of type <a class="reference internal" href="stdtypes.html#str" title="str"><code class="xref py py-class docutils literal"><span class="pre">str</span></code></a> and receives an <a class="reference internal" href="functions.html#int" title="int"><code class="xref py py-class docutils literal"><span class="pre">int</span></code></a> value at
runtime!</p>
<p>Furthermore, all functions without a return type or parameter types will
implicitly default to using <a class="reference internal" href="#typing.Any" title="typing.Any"><code class="xref py py-data docutils literal"><span class="pre">Any</span></code></a>:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">legacy_parser</span><span class="p">(</span><span class="n">text</span><span class="p">):</span>
<span class="o">...</span>
<span class="k">return</span> <span class="n">data</span>
<span class="c1"># A static type checker will treat the above</span>
<span class="c1"># as having the same signature as:</span>
<span class="k">def</span> <span class="nf">legacy_parser</span><span class="p">(</span><span class="n">text</span><span class="p">:</span> <span class="n">Any</span><span class="p">)</span> <span class="o">-></span> <span class="n">Any</span><span class="p">:</span>
<span class="o">...</span>
<span class="k">return</span> <span class="n">data</span>
</pre></div>
</div>
<p>This behavior allows <a class="reference internal" href="#typing.Any" title="typing.Any"><code class="xref py py-data docutils literal"><span class="pre">Any</span></code></a> to be used as an <em>escape hatch</em> when you
need to mix dynamically and statically typed code.</p>
<p>Contrast the behavior of <a class="reference internal" href="#typing.Any" title="typing.Any"><code class="xref py py-data docutils literal"><span class="pre">Any</span></code></a> with the behavior of <a class="reference internal" href="functions.html#object" title="object"><code class="xref py py-class docutils literal"><span class="pre">object</span></code></a>.
Similar to <a class="reference internal" href="#typing.Any" title="typing.Any"><code class="xref py py-data docutils literal"><span class="pre">Any</span></code></a>, every type is a subtype of <a class="reference internal" href="functions.html#object" title="object"><code class="xref py py-class docutils literal"><span class="pre">object</span></code></a>. However,
unlike <a class="reference internal" href="#typing.Any" title="typing.Any"><code class="xref py py-data docutils literal"><span class="pre">Any</span></code></a>, the reverse is not true: <a class="reference internal" href="functions.html#object" title="object"><code class="xref py py-class docutils literal"><span class="pre">object</span></code></a> is <em>not</em> a
subtype of every other type.</p>
<p>That means when the type of a value is <a class="reference internal" href="functions.html#object" title="object"><code class="xref py py-class docutils literal"><span class="pre">object</span></code></a>, a type checker will
reject almost all operations on it, and assigning it to a variable (or using
it as a return value) of a more specialized type is a type error. For example:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">hash_a</span><span class="p">(</span><span class="n">item</span><span class="p">:</span> <span class="nb">object</span><span class="p">)</span> <span class="o">-></span> <span class="nb">int</span><span class="p">:</span>
<span class="c1"># Fails; an object does not have a 'magic' method.</span>
<span class="n">item</span><span class="o">.</span><span class="n">magic</span><span class="p">()</span>
<span class="o">...</span>
<span class="k">def</span> <span class="nf">hash_b</span><span class="p">(</span><span class="n">item</span><span class="p">:</span> <span class="n">Any</span><span class="p">)</span> <span class="o">-></span> <span class="nb">int</span><span class="p">:</span>
<span class="c1"># Typechecks</span>
<span class="n">item</span><span class="o">.</span><span class="n">magic</span><span class="p">()</span>
<span class="o">...</span>
<span class="c1"># Typechecks, since ints and strs are subclasses of object</span>
<span class="n">hash_a</span><span class="p">(</span><span class="mi">42</span><span class="p">)</span>
<span class="n">hash_a</span><span class="p">(</span><span class="s2">"foo"</span><span class="p">)</span>
<span class="c1"># Typechecks, since Any is compatible with all types</span>
<span class="n">hash_b</span><span class="p">(</span><span class="mi">42</span><span class="p">)</span>
<span class="n">hash_b</span><span class="p">(</span><span class="s2">"foo"</span><span class="p">)</span>
</pre></div>
</div>
<p>Use <a class="reference internal" href="functions.html#object" title="object"><code class="xref py py-class docutils literal"><span class="pre">object</span></code></a> to indicate that a value could be any type in a typesafe
manner. Use <a class="reference internal" href="#typing.Any" title="typing.Any"><code class="xref py py-data docutils literal"><span class="pre">Any</span></code></a> to indicate that a value is dynamically typed.</p>
</div>
<div class="section" id="classes-functions-and-decorators">
<h2>26.1.7. Classes, functions, and decorators<a class="headerlink" href="#classes-functions-and-decorators" title="Permalink to this headline">¶</a></h2>
<p>The module defines the following classes, functions and decorators:</p>
<dl class="class">
<dt id="typing.TypeVar">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">TypeVar</code><a class="headerlink" href="#typing.TypeVar" title="Permalink to this definition">¶</a></dt>
<dd><p>Type variable.</p>
<p>Usage:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">T</span> <span class="o">=</span> <span class="n">TypeVar</span><span class="p">(</span><span class="s1">'T'</span><span class="p">)</span> <span class="c1"># Can be anything</span>
<span class="n">A</span> <span class="o">=</span> <span class="n">TypeVar</span><span class="p">(</span><span class="s1">'A'</span><span class="p">,</span> <span class="nb">str</span><span class="p">,</span> <span class="nb">bytes</span><span class="p">)</span> <span class="c1"># Must be str or bytes</span>
</pre></div>
</div>
<p>Type variables exist primarily for the benefit of static type
checkers. They serve as the parameters for generic types as well
as for generic function definitions. See class Generic for more
information on generic types. Generic functions work as follows:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">repeat</span><span class="p">(</span><span class="n">x</span><span class="p">:</span> <span class="n">T</span><span class="p">,</span> <span class="n">n</span><span class="p">:</span> <span class="nb">int</span><span class="p">)</span> <span class="o">-></span> <span class="n">Sequence</span><span class="p">[</span><span class="n">T</span><span class="p">]:</span>
<span class="sd">"""Return a list containing n references to x."""</span>
<span class="k">return</span> <span class="p">[</span><span class="n">x</span><span class="p">]</span><span class="o">*</span><span class="n">n</span>
<span class="k">def</span> <span class="nf">longest</span><span class="p">(</span><span class="n">x</span><span class="p">:</span> <span class="n">A</span><span class="p">,</span> <span class="n">y</span><span class="p">:</span> <span class="n">A</span><span class="p">)</span> <span class="o">-></span> <span class="n">A</span><span class="p">:</span>
<span class="sd">"""Return the longest of two strings."""</span>
<span class="k">return</span> <span class="n">x</span> <span class="k">if</span> <span class="nb">len</span><span class="p">(</span><span class="n">x</span><span class="p">)</span> <span class="o">>=</span> <span class="nb">len</span><span class="p">(</span><span class="n">y</span><span class="p">)</span> <span class="k">else</span> <span class="n">y</span>
</pre></div>
</div>
<p>The latter example’s signature is essentially the overloading
of <code class="docutils literal"><span class="pre">(str,</span> <span class="pre">str)</span> <span class="pre">-></span> <span class="pre">str</span></code> and <code class="docutils literal"><span class="pre">(bytes,</span> <span class="pre">bytes)</span> <span class="pre">-></span> <span class="pre">bytes</span></code>. Also note
that if the arguments are instances of some subclass of <a class="reference internal" href="stdtypes.html#str" title="str"><code class="xref py py-class docutils literal"><span class="pre">str</span></code></a>,
the return type is still plain <a class="reference internal" href="stdtypes.html#str" title="str"><code class="xref py py-class docutils literal"><span class="pre">str</span></code></a>.</p>
<p>At runtime, <code class="docutils literal"><span class="pre">isinstance(x,</span> <span class="pre">T)</span></code> will raise <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal"><span class="pre">TypeError</span></code></a>. In general,
<a class="reference internal" href="functions.html#isinstance" title="isinstance"><code class="xref py py-func docutils literal"><span class="pre">isinstance()</span></code></a> and <a class="reference internal" href="functions.html#issubclass" title="issubclass"><code class="xref py py-func docutils literal"><span class="pre">issubclass()</span></code></a> should not be used with types.</p>
<p>Type variables may be marked covariant or contravariant by passing
<code class="docutils literal"><span class="pre">covariant=True</span></code> or <code class="docutils literal"><span class="pre">contravariant=True</span></code>. See <span class="target" id="index-4"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0484"><strong>PEP 484</strong></a> for more
details. By default type variables are invariant. Alternatively,
a type variable may specify an upper bound using <code class="docutils literal"><span class="pre">bound=<type></span></code>.
This means that an actual type substituted (explicitly or implicitly)
for the type variable must be a subclass of the boundary type,
see <span class="target" id="index-5"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0484"><strong>PEP 484</strong></a>.</p>
</dd></dl>
<dl class="class">
<dt id="typing.Generic">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">Generic</code><a class="headerlink" href="#typing.Generic" title="Permalink to this definition">¶</a></dt>
<dd><p>Abstract base class for generic types.</p>
<p>A generic type is typically declared by inheriting from an
instantiation of this class with one or more type variables.
For example, a generic mapping type might be defined as:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Mapping</span><span class="p">(</span><span class="n">Generic</span><span class="p">[</span><span class="n">KT</span><span class="p">,</span> <span class="n">VT</span><span class="p">]):</span>
<span class="k">def</span> <span class="nf">__getitem__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">key</span><span class="p">:</span> <span class="n">KT</span><span class="p">)</span> <span class="o">-></span> <span class="n">VT</span><span class="p">:</span>
<span class="o">...</span>
<span class="c1"># Etc.</span>
</pre></div>
</div>
<p>This class can then be used as follows:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">X</span> <span class="o">=</span> <span class="n">TypeVar</span><span class="p">(</span><span class="s1">'X'</span><span class="p">)</span>
<span class="n">Y</span> <span class="o">=</span> <span class="n">TypeVar</span><span class="p">(</span><span class="s1">'Y'</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">lookup_name</span><span class="p">(</span><span class="n">mapping</span><span class="p">:</span> <span class="n">Mapping</span><span class="p">[</span><span class="n">X</span><span class="p">,</span> <span class="n">Y</span><span class="p">],</span> <span class="n">key</span><span class="p">:</span> <span class="n">X</span><span class="p">,</span> <span class="n">default</span><span class="p">:</span> <span class="n">Y</span><span class="p">)</span> <span class="o">-></span> <span class="n">Y</span><span class="p">:</span>
<span class="k">try</span><span class="p">:</span>
<span class="k">return</span> <span class="n">mapping</span><span class="p">[</span><span class="n">key</span><span class="p">]</span>
<span class="k">except</span> <span class="ne">KeyError</span><span class="p">:</span>
<span class="k">return</span> <span class="n">default</span>
</pre></div>
</div>
</dd></dl>
<dl class="class">
<dt id="typing.Type">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">Type</code><span class="sig-paren">(</span><em>Generic[CT_co]</em><span class="sig-paren">)</span><a class="headerlink" href="#typing.Type" title="Permalink to this definition">¶</a></dt>
<dd><p>A variable annotated with <code class="docutils literal"><span class="pre">C</span></code> may accept a value of type <code class="docutils literal"><span class="pre">C</span></code>. In
contrast, a variable annotated with <code class="docutils literal"><span class="pre">Type[C]</span></code> may accept values that are
classes themselves – specifically, it will accept the <em>class object</em> of
<code class="docutils literal"><span class="pre">C</span></code>. For example:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">a</span> <span class="o">=</span> <span class="mi">3</span> <span class="c1"># Has type 'int'</span>
<span class="n">b</span> <span class="o">=</span> <span class="nb">int</span> <span class="c1"># Has type 'Type[int]'</span>
<span class="n">c</span> <span class="o">=</span> <span class="nb">type</span><span class="p">(</span><span class="n">a</span><span class="p">)</span> <span class="c1"># Also has type 'Type[int]'</span>
</pre></div>
</div>
<p>Note that <code class="docutils literal"><span class="pre">Type[C]</span></code> is covariant:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">User</span><span class="p">:</span> <span class="o">...</span>
<span class="k">class</span> <span class="nc">BasicUser</span><span class="p">(</span><span class="n">User</span><span class="p">):</span> <span class="o">...</span>
<span class="k">class</span> <span class="nc">ProUser</span><span class="p">(</span><span class="n">User</span><span class="p">):</span> <span class="o">...</span>
<span class="k">class</span> <span class="nc">TeamUser</span><span class="p">(</span><span class="n">User</span><span class="p">):</span> <span class="o">...</span>
<span class="c1"># Accepts User, BasicUser, ProUser, TeamUser, ...</span>
<span class="k">def</span> <span class="nf">make_new_user</span><span class="p">(</span><span class="n">user_class</span><span class="p">:</span> <span class="n">Type</span><span class="p">[</span><span class="n">User</span><span class="p">])</span> <span class="o">-></span> <span class="n">User</span><span class="p">:</span>
<span class="c1"># ...</span>
<span class="k">return</span> <span class="n">user_class</span><span class="p">()</span>
</pre></div>
</div>
<p>The fact that <code class="docutils literal"><span class="pre">Type[C]</span></code> is covariant implies that all subclasses of
<code class="docutils literal"><span class="pre">C</span></code> should implement the same constructor signature and class method
signatures as <code class="docutils literal"><span class="pre">C</span></code>. The type checker should flag violations of this,
but should also allow constructor calls in subclasses that match the
constructor calls in the indicated base class. How the type checker is
required to handle this particular case may change in future revisions of
<span class="target" id="index-6"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0484"><strong>PEP 484</strong></a>.</p>
<p>The only legal parameters for <a class="reference internal" href="#typing.Type" title="typing.Type"><code class="xref py py-class docutils literal"><span class="pre">Type</span></code></a> are classes, unions of classes, and
<a class="reference internal" href="#typing.Any" title="typing.Any"><code class="xref py py-data docutils literal"><span class="pre">Any</span></code></a>. For example:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">new_non_team_user</span><span class="p">(</span><span class="n">user_class</span><span class="p">:</span> <span class="n">Type</span><span class="p">[</span><span class="n">Union</span><span class="p">[</span><span class="n">BaseUser</span><span class="p">,</span> <span class="n">ProUser</span><span class="p">]]):</span> <span class="o">...</span>
</pre></div>
</div>
<p><code class="docutils literal"><span class="pre">Type[Any]</span></code> is equivalent to <code class="docutils literal"><span class="pre">Type</span></code> which in turn is equivalent
to <code class="docutils literal"><span class="pre">type</span></code>, which is the root of Python’s metaclass hierarchy.</p>
</dd></dl>
<dl class="class">
<dt id="typing.Iterable">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">Iterable</code><span class="sig-paren">(</span><em>Generic[T_co]</em><span class="sig-paren">)</span><a class="headerlink" href="#typing.Iterable" title="Permalink to this definition">¶</a></dt>
<dd><p>A generic version of <a class="reference internal" href="collections.abc.html#collections.abc.Iterable" title="collections.abc.Iterable"><code class="xref py py-class docutils literal"><span class="pre">collections.abc.Iterable</span></code></a>.</p>
</dd></dl>
<dl class="class">
<dt id="typing.Iterator">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">Iterator</code><span class="sig-paren">(</span><em>Iterable[T_co]</em><span class="sig-paren">)</span><a class="headerlink" href="#typing.Iterator" title="Permalink to this definition">¶</a></dt>
<dd><p>A generic version of <a class="reference internal" href="collections.abc.html#collections.abc.Iterator" title="collections.abc.Iterator"><code class="xref py py-class docutils literal"><span class="pre">collections.abc.Iterator</span></code></a>.</p>
</dd></dl>
<dl class="class">
<dt id="typing.Reversible">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">Reversible</code><span class="sig-paren">(</span><em>Iterable[T_co]</em><span class="sig-paren">)</span><a class="headerlink" href="#typing.Reversible" title="Permalink to this definition">¶</a></dt>
<dd><p>A generic version of <code class="xref py py-class docutils literal"><span class="pre">collections.abc.Reversible</span></code>.</p>
</dd></dl>
<dl class="class">
<dt id="typing.SupportsInt">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">SupportsInt</code><a class="headerlink" href="#typing.SupportsInt" title="Permalink to this definition">¶</a></dt>
<dd><p>An ABC with one abstract method <code class="docutils literal"><span class="pre">__int__</span></code>.</p>
</dd></dl>
<dl class="class">
<dt id="typing.SupportsFloat">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">SupportsFloat</code><a class="headerlink" href="#typing.SupportsFloat" title="Permalink to this definition">¶</a></dt>
<dd><p>An ABC with one abstract method <code class="docutils literal"><span class="pre">__float__</span></code>.</p>
</dd></dl>
<dl class="class">
<dt id="typing.SupportsAbs">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">SupportsAbs</code><a class="headerlink" href="#typing.SupportsAbs" title="Permalink to this definition">¶</a></dt>
<dd><p>An ABC with one abstract method <code class="docutils literal"><span class="pre">__abs__</span></code> that is covariant
in its return type.</p>
</dd></dl>
<dl class="class">
<dt id="typing.SupportsRound">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">SupportsRound</code><a class="headerlink" href="#typing.SupportsRound" title="Permalink to this definition">¶</a></dt>
<dd><p>An ABC with one abstract method <code class="docutils literal"><span class="pre">__round__</span></code>
that is covariant in its return type.</p>
</dd></dl>
<dl class="class">
<dt id="typing.Container">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">Container</code><span class="sig-paren">(</span><em>Generic[T_co]</em><span class="sig-paren">)</span><a class="headerlink" href="#typing.Container" title="Permalink to this definition">¶</a></dt>
<dd><p>A generic version of <a class="reference internal" href="collections.abc.html#collections.abc.Container" title="collections.abc.Container"><code class="xref py py-class docutils literal"><span class="pre">collections.abc.Container</span></code></a>.</p>
</dd></dl>
<dl class="class">
<dt id="typing.Hashable">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">Hashable</code><a class="headerlink" href="#typing.Hashable" title="Permalink to this definition">¶</a></dt>
<dd><p>An alias to <a class="reference internal" href="collections.abc.html#collections.abc.Hashable" title="collections.abc.Hashable"><code class="xref py py-class docutils literal"><span class="pre">collections.abc.Hashable</span></code></a></p>
</dd></dl>
<dl class="class">
<dt id="typing.Sized">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">Sized</code><a class="headerlink" href="#typing.Sized" title="Permalink to this definition">¶</a></dt>
<dd><p>An alias to <a class="reference internal" href="collections.abc.html#collections.abc.Sized" title="collections.abc.Sized"><code class="xref py py-class docutils literal"><span class="pre">collections.abc.Sized</span></code></a></p>
</dd></dl>
<dl class="class">
<dt id="typing.AbstractSet">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">AbstractSet</code><span class="sig-paren">(</span><em>Sized, Iterable[T_co], Container[T_co]</em><span class="sig-paren">)</span><a class="headerlink" href="#typing.AbstractSet" title="Permalink to this definition">¶</a></dt>
<dd><p>A generic version of <a class="reference internal" href="collections.abc.html#collections.abc.Set" title="collections.abc.Set"><code class="xref py py-class docutils literal"><span class="pre">collections.abc.Set</span></code></a>.</p>
</dd></dl>
<dl class="class">
<dt id="typing.MutableSet">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">MutableSet</code><span class="sig-paren">(</span><em>AbstractSet[T]</em><span class="sig-paren">)</span><a class="headerlink" href="#typing.MutableSet" title="Permalink to this definition">¶</a></dt>
<dd><p>A generic version of <a class="reference internal" href="collections.abc.html#collections.abc.MutableSet" title="collections.abc.MutableSet"><code class="xref py py-class docutils literal"><span class="pre">collections.abc.MutableSet</span></code></a>.</p>
</dd></dl>
<dl class="class">
<dt id="typing.Mapping">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">Mapping</code><span class="sig-paren">(</span><em>Sized, Iterable[KT], Container[KT], Generic[VT_co]</em><span class="sig-paren">)</span><a class="headerlink" href="#typing.Mapping" title="Permalink to this definition">¶</a></dt>
<dd><p>A generic version of <a class="reference internal" href="collections.abc.html#collections.abc.Mapping" title="collections.abc.Mapping"><code class="xref py py-class docutils literal"><span class="pre">collections.abc.Mapping</span></code></a>.</p>
</dd></dl>
<dl class="class">
<dt id="typing.MutableMapping">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">MutableMapping</code><span class="sig-paren">(</span><em>Mapping[KT, VT]</em><span class="sig-paren">)</span><a class="headerlink" href="#typing.MutableMapping" title="Permalink to this definition">¶</a></dt>
<dd><p>A generic version of <a class="reference internal" href="collections.abc.html#collections.abc.MutableMapping" title="collections.abc.MutableMapping"><code class="xref py py-class docutils literal"><span class="pre">collections.abc.MutableMapping</span></code></a>.</p>
</dd></dl>
<dl class="class">
<dt id="typing.Sequence">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">Sequence</code><span class="sig-paren">(</span><em>Sized, Iterable[T_co], Container[T_co]</em><span class="sig-paren">)</span><a class="headerlink" href="#typing.Sequence" title="Permalink to this definition">¶</a></dt>
<dd><p>A generic version of <a class="reference internal" href="collections.abc.html#collections.abc.Sequence" title="collections.abc.Sequence"><code class="xref py py-class docutils literal"><span class="pre">collections.abc.Sequence</span></code></a>.</p>
</dd></dl>
<dl class="class">
<dt id="typing.MutableSequence">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">MutableSequence</code><span class="sig-paren">(</span><em>Sequence[T]</em><span class="sig-paren">)</span><a class="headerlink" href="#typing.MutableSequence" title="Permalink to this definition">¶</a></dt>
<dd><p>A generic version of <a class="reference internal" href="collections.abc.html#collections.abc.MutableSequence" title="collections.abc.MutableSequence"><code class="xref py py-class docutils literal"><span class="pre">collections.abc.MutableSequence</span></code></a>.</p>
</dd></dl>
<dl class="class">
<dt id="typing.ByteString">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">ByteString</code><span class="sig-paren">(</span><em>Sequence[int]</em><span class="sig-paren">)</span><a class="headerlink" href="#typing.ByteString" title="Permalink to this definition">¶</a></dt>
<dd><p>A generic version of <a class="reference internal" href="collections.abc.html#collections.abc.ByteString" title="collections.abc.ByteString"><code class="xref py py-class docutils literal"><span class="pre">collections.abc.ByteString</span></code></a>.</p>
<p>This type represents the types <a class="reference internal" href="functions.html#bytes" title="bytes"><code class="xref py py-class docutils literal"><span class="pre">bytes</span></code></a>, <a class="reference internal" href="functions.html#bytearray" title="bytearray"><code class="xref py py-class docutils literal"><span class="pre">bytearray</span></code></a>,
and <a class="reference internal" href="stdtypes.html#memoryview" title="memoryview"><code class="xref py py-class docutils literal"><span class="pre">memoryview</span></code></a>.</p>
<p>As a shorthand for this type, <a class="reference internal" href="functions.html#bytes" title="bytes"><code class="xref py py-class docutils literal"><span class="pre">bytes</span></code></a> can be used to
annotate arguments of any of the types mentioned above.</p>
</dd></dl>
<dl class="class">
<dt id="typing.Deque">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">Deque</code><span class="sig-paren">(</span><em>deque, MutableSequence[T]</em><span class="sig-paren">)</span><a class="headerlink" href="#typing.Deque" title="Permalink to this definition">¶</a></dt>
<dd><p>A generic version of <a class="reference internal" href="collections.html#collections.deque" title="collections.deque"><code class="xref py py-class docutils literal"><span class="pre">collections.deque</span></code></a>.</p>
</dd></dl>
<dl class="class">
<dt id="typing.List">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">List</code><span class="sig-paren">(</span><em>list, MutableSequence[T]</em><span class="sig-paren">)</span><a class="headerlink" href="#typing.List" title="Permalink to this definition">¶</a></dt>
<dd><p>Generic version of <a class="reference internal" href="stdtypes.html#list" title="list"><code class="xref py py-class docutils literal"><span class="pre">list</span></code></a>.
Useful for annotating return types. To annotate arguments it is preferred
to use abstract collection types such as <a class="reference internal" href="#typing.Mapping" title="typing.Mapping"><code class="xref py py-class docutils literal"><span class="pre">Mapping</span></code></a>, <a class="reference internal" href="#typing.Sequence" title="typing.Sequence"><code class="xref py py-class docutils literal"><span class="pre">Sequence</span></code></a>,
or <a class="reference internal" href="#typing.AbstractSet" title="typing.AbstractSet"><code class="xref py py-class docutils literal"><span class="pre">AbstractSet</span></code></a>.</p>
<p>This type may be used as follows:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">T</span> <span class="o">=</span> <span class="n">TypeVar</span><span class="p">(</span><span class="s1">'T'</span><span class="p">,</span> <span class="nb">int</span><span class="p">,</span> <span class="nb">float</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">vec2</span><span class="p">(</span><span class="n">x</span><span class="p">:</span> <span class="n">T</span><span class="p">,</span> <span class="n">y</span><span class="p">:</span> <span class="n">T</span><span class="p">)</span> <span class="o">-></span> <span class="n">List</span><span class="p">[</span><span class="n">T</span><span class="p">]:</span>
<span class="k">return</span> <span class="p">[</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">]</span>
<span class="k">def</span> <span class="nf">keep_positives</span><span class="p">(</span><span class="n">vector</span><span class="p">:</span> <span class="n">Sequence</span><span class="p">[</span><span class="n">T</span><span class="p">])</span> <span class="o">-></span> <span class="n">List</span><span class="p">[</span><span class="n">T</span><span class="p">]:</span>
<span class="k">return</span> <span class="p">[</span><span class="n">item</span> <span class="k">for</span> <span class="n">item</span> <span class="ow">in</span> <span class="n">vector</span> <span class="k">if</span> <span class="n">item</span> <span class="o">></span> <span class="mi">0</span><span class="p">]</span>
</pre></div>
</div>
</dd></dl>
<dl class="class">
<dt id="typing.Set">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">Set</code><span class="sig-paren">(</span><em>set, MutableSet[T]</em><span class="sig-paren">)</span><a class="headerlink" href="#typing.Set" title="Permalink to this definition">¶</a></dt>
<dd><p>A generic version of <a class="reference internal" href="stdtypes.html#set" title="set"><code class="xref py py-class docutils literal"><span class="pre">builtins.set</span></code></a>.</p>
</dd></dl>
<dl class="class">
<dt id="typing.FrozenSet">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">FrozenSet</code><span class="sig-paren">(</span><em>frozenset, AbstractSet[T_co]</em><span class="sig-paren">)</span><a class="headerlink" href="#typing.FrozenSet" title="Permalink to this definition">¶</a></dt>
<dd><p>A generic version of <a class="reference internal" href="stdtypes.html#frozenset" title="frozenset"><code class="xref py py-class docutils literal"><span class="pre">builtins.frozenset</span></code></a>.</p>
</dd></dl>
<dl class="class">
<dt id="typing.MappingView">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">MappingView</code><span class="sig-paren">(</span><em>Sized, Iterable[T_co]</em><span class="sig-paren">)</span><a class="headerlink" href="#typing.MappingView" title="Permalink to this definition">¶</a></dt>
<dd><p>A generic version of <a class="reference internal" href="collections.abc.html#collections.abc.MappingView" title="collections.abc.MappingView"><code class="xref py py-class docutils literal"><span class="pre">collections.abc.MappingView</span></code></a>.</p>
</dd></dl>
<dl class="class">
<dt id="typing.KeysView">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">KeysView</code><span class="sig-paren">(</span><em>MappingView[KT_co], AbstractSet[KT_co]</em><span class="sig-paren">)</span><a class="headerlink" href="#typing.KeysView" title="Permalink to this definition">¶</a></dt>
<dd><p>A generic version of <a class="reference internal" href="collections.abc.html#collections.abc.KeysView" title="collections.abc.KeysView"><code class="xref py py-class docutils literal"><span class="pre">collections.abc.KeysView</span></code></a>.</p>
</dd></dl>
<dl class="class">
<dt id="typing.ItemsView">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">ItemsView</code><span class="sig-paren">(</span><em>MappingView, Generic[KT_co, VT_co]</em><span class="sig-paren">)</span><a class="headerlink" href="#typing.ItemsView" title="Permalink to this definition">¶</a></dt>
<dd><p>A generic version of <a class="reference internal" href="collections.abc.html#collections.abc.ItemsView" title="collections.abc.ItemsView"><code class="xref py py-class docutils literal"><span class="pre">collections.abc.ItemsView</span></code></a>.</p>
</dd></dl>
<dl class="class">
<dt id="typing.ValuesView">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">ValuesView</code><span class="sig-paren">(</span><em>MappingView[VT_co]</em><span class="sig-paren">)</span><a class="headerlink" href="#typing.ValuesView" title="Permalink to this definition">¶</a></dt>
<dd><p>A generic version of <a class="reference internal" href="collections.abc.html#collections.abc.ValuesView" title="collections.abc.ValuesView"><code class="xref py py-class docutils literal"><span class="pre">collections.abc.ValuesView</span></code></a>.</p>
</dd></dl>
<dl class="class">
<dt id="typing.Awaitable">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">Awaitable</code><span class="sig-paren">(</span><em>Generic[T_co]</em><span class="sig-paren">)</span><a class="headerlink" href="#typing.Awaitable" title="Permalink to this definition">¶</a></dt>
<dd><p>A generic version of <a class="reference internal" href="collections.abc.html#collections.abc.Awaitable" title="collections.abc.Awaitable"><code class="xref py py-class docutils literal"><span class="pre">collections.abc.Awaitable</span></code></a>.</p>
</dd></dl>
<dl class="class">
<dt id="typing.Coroutine">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">Coroutine</code><span class="sig-paren">(</span><em>Awaitable[V_co], Generic[T_co T_contra, V_co]</em><span class="sig-paren">)</span><a class="headerlink" href="#typing.Coroutine" title="Permalink to this definition">¶</a></dt>
<dd><p>A generic version of <a class="reference internal" href="collections.abc.html#collections.abc.Coroutine" title="collections.abc.Coroutine"><code class="xref py py-class docutils literal"><span class="pre">collections.abc.Coroutine</span></code></a>.
The variance and order of type variables
correspond to those of <a class="reference internal" href="#typing.Generator" title="typing.Generator"><code class="xref py py-class docutils literal"><span class="pre">Generator</span></code></a>, for example:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">typing</span> <span class="k">import</span> <span class="n">List</span><span class="p">,</span> <span class="n">Coroutine</span>
<span class="n">c</span> <span class="o">=</span> <span class="kc">None</span> <span class="c1"># type: Coroutine[List[str], str, int]</span>
<span class="o">...</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">c</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="s1">'hi'</span><span class="p">)</span> <span class="c1"># type: List[str]</span>
<span class="k">async</span> <span class="k">def</span> <span class="nf">bar</span><span class="p">()</span> <span class="o">-></span> <span class="kc">None</span><span class="p">:</span>
<span class="n">x</span> <span class="o">=</span> <span class="k">await</span> <span class="n">c</span> <span class="c1"># type: int</span>
</pre></div>
</div>
</dd></dl>
<dl class="class">
<dt id="typing.AsyncIterable">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">AsyncIterable</code><span class="sig-paren">(</span><em>Generic[T_co]</em><span class="sig-paren">)</span><a class="headerlink" href="#typing.AsyncIterable" title="Permalink to this definition">¶</a></dt>
<dd><p>A generic version of <a class="reference internal" href="collections.abc.html#collections.abc.AsyncIterable" title="collections.abc.AsyncIterable"><code class="xref py py-class docutils literal"><span class="pre">collections.abc.AsyncIterable</span></code></a>.</p>
</dd></dl>
<dl class="class">
<dt id="typing.AsyncIterator">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">AsyncIterator</code><span class="sig-paren">(</span><em>AsyncIterable[T_co]</em><span class="sig-paren">)</span><a class="headerlink" href="#typing.AsyncIterator" title="Permalink to this definition">¶</a></dt>
<dd><p>A generic version of <a class="reference internal" href="collections.abc.html#collections.abc.AsyncIterator" title="collections.abc.AsyncIterator"><code class="xref py py-class docutils literal"><span class="pre">collections.abc.AsyncIterator</span></code></a>.</p>
</dd></dl>
<dl class="class">
<dt id="typing.Dict">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">Dict</code><span class="sig-paren">(</span><em>dict, MutableMapping[KT, VT]</em><span class="sig-paren">)</span><a class="headerlink" href="#typing.Dict" title="Permalink to this definition">¶</a></dt>
<dd><p>A generic version of <a class="reference internal" href="stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal"><span class="pre">dict</span></code></a>.
The usage of this type is as follows:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">get_position_in_index</span><span class="p">(</span><span class="n">word_list</span><span class="p">:</span> <span class="n">Dict</span><span class="p">[</span><span class="nb">str</span><span class="p">,</span> <span class="nb">int</span><span class="p">],</span> <span class="n">word</span><span class="p">:</span> <span class="nb">str</span><span class="p">)</span> <span class="o">-></span> <span class="nb">int</span><span class="p">:</span>
<span class="k">return</span> <span class="n">word_list</span><span class="p">[</span><span class="n">word</span><span class="p">]</span>
</pre></div>
</div>
</dd></dl>
<dl class="class">
<dt id="typing.DefaultDict">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">DefaultDict</code><span class="sig-paren">(</span><em>collections.defaultdict, MutableMapping[KT, VT]</em><span class="sig-paren">)</span><a class="headerlink" href="#typing.DefaultDict" title="Permalink to this definition">¶</a></dt>
<dd><p>A generic version of <a class="reference internal" href="collections.html#collections.defaultdict" title="collections.defaultdict"><code class="xref py py-class docutils literal"><span class="pre">collections.defaultdict</span></code></a></p>
</dd></dl>
<dl class="class">
<dt id="typing.Generator">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">Generator</code><span class="sig-paren">(</span><em>Iterator[T_co], Generic[T_co, T_contra, V_co]</em><span class="sig-paren">)</span><a class="headerlink" href="#typing.Generator" title="Permalink to this definition">¶</a></dt>
<dd><p>A generator can be annotated by the generic type
<code class="docutils literal"><span class="pre">Generator[YieldType,</span> <span class="pre">SendType,</span> <span class="pre">ReturnType]</span></code>. For example:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">echo_round</span><span class="p">()</span> <span class="o">-></span> <span class="n">Generator</span><span class="p">[</span><span class="nb">int</span><span class="p">,</span> <span class="nb">float</span><span class="p">,</span> <span class="nb">str</span><span class="p">]:</span>
<span class="n">sent</span> <span class="o">=</span> <span class="k">yield</span> <span class="mi">0</span>
<span class="k">while</span> <span class="n">sent</span> <span class="o">>=</span> <span class="mi">0</span><span class="p">:</span>
<span class="n">sent</span> <span class="o">=</span> <span class="k">yield</span> <span class="nb">round</span><span class="p">(</span><span class="n">sent</span><span class="p">)</span>
<span class="k">return</span> <span class="s1">'Done'</span>
</pre></div>
</div>
<p>Note that unlike many other generics in the typing module, the <code class="docutils literal"><span class="pre">SendType</span></code>
of <a class="reference internal" href="#typing.Generator" title="typing.Generator"><code class="xref py py-class docutils literal"><span class="pre">Generator</span></code></a> behaves contravariantly, not covariantly or
invariantly.</p>
<p>If your generator will only yield values, set the <code class="docutils literal"><span class="pre">SendType</span></code> and
<code class="docutils literal"><span class="pre">ReturnType</span></code> to <code class="docutils literal"><span class="pre">None</span></code>:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">infinite_stream</span><span class="p">(</span><span class="n">start</span><span class="p">:</span> <span class="nb">int</span><span class="p">)</span> <span class="o">-></span> <span class="n">Generator</span><span class="p">[</span><span class="nb">int</span><span class="p">,</span> <span class="kc">None</span><span class="p">,</span> <span class="kc">None</span><span class="p">]:</span>
<span class="k">while</span> <span class="kc">True</span><span class="p">:</span>
<span class="k">yield</span> <span class="n">start</span>
<span class="n">start</span> <span class="o">+=</span> <span class="mi">1</span>
</pre></div>
</div>
<p>Alternatively, annotate your generator as having a return type of
either <code class="docutils literal"><span class="pre">Iterable[YieldType]</span></code> or <code class="docutils literal"><span class="pre">Iterator[YieldType]</span></code>:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">infinite_stream</span><span class="p">(</span><span class="n">start</span><span class="p">:</span> <span class="nb">int</span><span class="p">)</span> <span class="o">-></span> <span class="n">Iterator</span><span class="p">[</span><span class="nb">int</span><span class="p">]:</span>
<span class="k">while</span> <span class="kc">True</span><span class="p">:</span>
<span class="k">yield</span> <span class="n">start</span>
<span class="n">start</span> <span class="o">+=</span> <span class="mi">1</span>
</pre></div>
</div>
</dd></dl>
<dl class="class">
<dt id="typing.Text">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">Text</code><a class="headerlink" href="#typing.Text" title="Permalink to this definition">¶</a></dt>
<dd><p><code class="docutils literal"><span class="pre">Text</span></code> is an alias for <code class="docutils literal"><span class="pre">str</span></code>. It is provided to supply a forward
compatible path for Python 2 code: in Python 2, <code class="docutils literal"><span class="pre">Text</span></code> is an alias for
<code class="docutils literal"><span class="pre">unicode</span></code>.</p>
<p>Use <code class="docutils literal"><span class="pre">Text</span></code> to indicate that a value must contain a unicode string in
a manner that is compatible with both Python 2 and Python 3:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">add_unicode_checkmark</span><span class="p">(</span><span class="n">text</span><span class="p">:</span> <span class="n">Text</span><span class="p">)</span> <span class="o">-></span> <span class="n">Text</span><span class="p">:</span>
<span class="k">return</span> <span class="n">text</span> <span class="o">+</span> <span class="s1">u' </span><span class="se">\u2713</span><span class="s1">'</span>
</pre></div>
</div>
</dd></dl>
<dl class="class">
<dt id="typing.io">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">io</code><a class="headerlink" href="#typing.io" title="Permalink to this definition">¶</a></dt>
<dd><p>Wrapper namespace for I/O stream types.</p>
<p>This defines the generic type <code class="docutils literal"><span class="pre">IO[AnyStr]</span></code> and aliases <code class="docutils literal"><span class="pre">TextIO</span></code>
and <code class="docutils literal"><span class="pre">BinaryIO</span></code> for respectively <code class="docutils literal"><span class="pre">IO[str]</span></code> and <code class="docutils literal"><span class="pre">IO[bytes]</span></code>.
These representing the types of I/O streams such as returned by
<a class="reference internal" href="functions.html#open" title="open"><code class="xref py py-func docutils literal"><span class="pre">open()</span></code></a>.</p>
</dd></dl>
<dl class="class">
<dt id="typing.re">
<em class="property">class </em><code class="descclassname">typing.</code><code class="descname">re</code><a class="headerlink" href="#typing.re" title="Permalink to this definition">¶</a></dt>
<dd><p>Wrapper namespace for regular expression matching types.</p>
<p>This defines the type aliases <code class="docutils literal"><span class="pre">Pattern</span></code> and <code class="docutils literal"><span class="pre">Match</span></code> which
correspond to the return types from <a class="reference internal" href="re.html#re.compile" title="re.compile"><code class="xref py py-func docutils literal"><span class="pre">re.compile()</span></code></a> and
<a class="reference internal" href="re.html#re.match" title="re.match"><code class="xref py py-func docutils literal"><span class="pre">re.match()</span></code></a>. These types (and the corresponding functions)
are generic in <code class="docutils literal"><span class="pre">AnyStr</span></code> and can be made specific by writing
<code class="docutils literal"><span class="pre">Pattern[str]</span></code>, <code class="docutils literal"><span class="pre">Pattern[bytes]</span></code>, <code class="docutils literal"><span class="pre">Match[str]</span></code>, or
<code class="docutils literal"><span class="pre">Match[bytes]</span></code>.</p>
</dd></dl>
<dl class="function">
<dt id="typing.NamedTuple">
<code class="descclassname">typing.</code><code class="descname">NamedTuple</code><span class="sig-paren">(</span><em>typename</em>, <em>fields</em><span class="sig-paren">)</span><a class="headerlink" href="#typing.NamedTuple" title="Permalink to this definition">¶</a></dt>
<dd><p>Typed version of namedtuple.</p>
<p>Usage:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">Employee</span> <span class="o">=</span> <span class="n">typing</span><span class="o">.</span><span class="n">NamedTuple</span><span class="p">(</span><span class="s1">'Employee'</span><span class="p">,</span> <span class="p">[(</span><span class="s1">'name'</span><span class="p">,</span> <span class="nb">str</span><span class="p">),</span> <span class="p">(</span><span class="s1">'id'</span><span class="p">,</span> <span class="nb">int</span><span class="p">)])</span>
</pre></div>
</div>
<p>This is equivalent to:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">Employee</span> <span class="o">=</span> <span class="n">collections</span><span class="o">.</span><span class="n">namedtuple</span><span class="p">(</span><span class="s1">'Employee'</span><span class="p">,</span> <span class="p">[</span><span class="s1">'name'</span><span class="p">,</span> <span class="s1">'id'</span><span class="p">])</span>
</pre></div>
</div>
<p>The resulting class has one extra attribute: _field_types,
giving a dict mapping field names to types. (The field names
are in the _fields attribute, which is part of the namedtuple
API.)</p>
</dd></dl>
<dl class="function">
<dt id="typing.NewType">
<code class="descclassname">typing.</code><code class="descname">NewType</code><span class="sig-paren">(</span><em>typ</em><span class="sig-paren">)</span><a class="headerlink" href="#typing.NewType" title="Permalink to this definition">¶</a></dt>
<dd><p>A helper function to indicate a distinct types to a typechecker,
see <a class="reference internal" href="#distinct"><span>NewType</span></a>. At runtime it returns a function that returns
its argument. Usage:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">UserId</span> <span class="o">=</span> <span class="n">NewType</span><span class="p">(</span><span class="s1">'UserId'</span><span class="p">,</span> <span class="nb">int</span><span class="p">)</span>
<span class="n">first_user</span> <span class="o">=</span> <span class="n">UserId</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
</pre></div>
</div>
</dd></dl>
<dl class="function">
<dt id="typing.cast">
<code class="descclassname">typing.</code><code class="descname">cast</code><span class="sig-paren">(</span><em>typ</em>, <em>val</em><span class="sig-paren">)</span><a class="headerlink" href="#typing.cast" title="Permalink to this definition">¶</a></dt>
<dd><p>Cast a value to a type.</p>
<p>This returns the value unchanged. To the type checker this
signals that the return value has the designated type, but at
runtime we intentionally don’t check anything (we want this
to be as fast as possible).</p>
</dd></dl>
<dl class="function">
<dt id="typing.get_type_hints">
<code class="descclassname">typing.</code><code class="descname">get_type_hints</code><span class="sig-paren">(</span><em>obj</em><span class="optional">[</span>, <em>globals</em><span class="optional">[</span>, <em>locals</em><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#typing.get_type_hints" title="Permalink to this definition">¶</a></dt>
<dd><p>Return a dictionary containing type hints for a function, method, module
or class object.</p>
<p>This is often the same as <code class="docutils literal"><span class="pre">obj.__annotations__</span></code>. In addition,
forward references encoded as string literals are handled by evaluating
them in <code class="docutils literal"><span class="pre">globals</span></code> and <code class="docutils literal"><span class="pre">locals</span></code> namespaces. If necessary,
<code class="docutils literal"><span class="pre">Optional[t]</span></code> is added for function and method annotations if a default
value equal to <code class="docutils literal"><span class="pre">None</span></code> is set. For a class <code class="docutils literal"><span class="pre">C</span></code>, return
a dictionary constructed by merging all the <code class="docutils literal"><span class="pre">__annotations__</span></code> along
<code class="docutils literal"><span class="pre">C.__mro__</span></code> in reverse order.</p>
</dd></dl>
<dl class="function">
<dt id="typing.overload">
<code class="descclassname">@</code><code class="descclassname">typing.</code><code class="descname">overload</code><a class="headerlink" href="#typing.overload" title="Permalink to this definition">¶</a></dt>
<dd><p>The <code class="docutils literal"><span class="pre">@overload</span></code> decorator allows describing functions and methods
that support multiple different combinations of argument types. A series
of <code class="docutils literal"><span class="pre">@overload</span></code>-decorated definitions must be followed by exactly one
non-<code class="docutils literal"><span class="pre">@overload</span></code>-decorated definition (for the same function/method).
The <code class="docutils literal"><span class="pre">@overload</span></code>-decorated definitions are for the benefit of the
type checker only, since they will be overwritten by the
non-<code class="docutils literal"><span class="pre">@overload</span></code>-decorated definition, while the latter is used at
runtime but should be ignored by a type checker. At runtime, calling
a <code class="docutils literal"><span class="pre">@overload</span></code>-decorated function directly will raise
<code class="docutils literal"><span class="pre">NotImplementedError</span></code>. An example of overload that gives a more
precise type than can be expressed using a union or a type variable:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="nd">@overload</span>
<span class="k">def</span> <span class="nf">process</span><span class="p">(</span><span class="n">response</span><span class="p">:</span> <span class="kc">None</span><span class="p">)</span> <span class="o">-></span> <span class="kc">None</span><span class="p">:</span>
<span class="o">...</span>
<span class="nd">@overload</span>
<span class="k">def</span> <span class="nf">process</span><span class="p">(</span><span class="n">response</span><span class="p">:</span> <span class="nb">int</span><span class="p">)</span> <span class="o">-></span> <span class="n">Tuple</span><span class="p">[</span><span class="nb">int</span><span class="p">,</span> <span class="nb">str</span><span class="p">]:</span>
<span class="o">...</span>
<span class="nd">@overload</span>
<span class="k">def</span> <span class="nf">process</span><span class="p">(</span><span class="n">response</span><span class="p">:</span> <span class="nb">bytes</span><span class="p">)</span> <span class="o">-></span> <span class="nb">str</span><span class="p">:</span>
<span class="o">...</span>
<span class="k">def</span> <span class="nf">process</span><span class="p">(</span><span class="n">response</span><span class="p">):</span>
<span class="o"><</span><span class="n">actual</span> <span class="n">implementation</span><span class="o">></span>
</pre></div>
</div>
<p>See <span class="target" id="index-7"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0484"><strong>PEP 484</strong></a> for details and comparison with other typing semantics.</p>
</dd></dl>
<dl class="function">
<dt id="typing.no_type_check">
<code class="descclassname">@</code><code class="descclassname">typing.</code><code class="descname">no_type_check</code><span class="sig-paren">(</span><em>arg</em><span class="sig-paren">)</span><a class="headerlink" href="#typing.no_type_check" title="Permalink to this definition">¶</a></dt>
<dd><p>Decorator to indicate that annotations are not type hints.</p>
<p>The argument must be a class or function; if it is a class, it
applies recursively to all methods defined in that class (but not
to methods defined in its superclasses or subclasses).</p>
<p>This mutates the function(s) in place.</p>
</dd></dl>
<dl class="function">
<dt id="typing.no_type_check_decorator">
<code class="descclassname">@</code><code class="descclassname">typing.</code><code class="descname">no_type_check_decorator</code><span class="sig-paren">(</span><em>decorator</em><span class="sig-paren">)</span><a class="headerlink" href="#typing.no_type_check_decorator" title="Permalink to this definition">¶</a></dt>
<dd><p>Decorator to give another decorator the <a class="reference internal" href="#typing.no_type_check" title="typing.no_type_check"><code class="xref py py-func docutils literal"><span class="pre">no_type_check()</span></code></a> effect.</p>
<p>This wraps the decorator with something that wraps the decorated
function in <a class="reference internal" href="#typing.no_type_check" title="typing.no_type_check"><code class="xref py py-func docutils literal"><span class="pre">no_type_check()</span></code></a>.</p>
</dd></dl>
<dl class="data">
<dt id="typing.Any">
<code class="descclassname">typing.</code><code class="descname">Any</code><a class="headerlink" href="#typing.Any" title="Permalink to this definition">¶</a></dt>
<dd><p>Special type indicating an unconstrained type.</p>
<ul class="simple">
<li>Every type is compatible with <a class="reference internal" href="#typing.Any" title="typing.Any"><code class="xref py py-data docutils literal"><span class="pre">Any</span></code></a>.</li>
<li><a class="reference internal" href="#typing.Any" title="typing.Any"><code class="xref py py-data docutils literal"><span class="pre">Any</span></code></a> is compatible with every type.</li>
</ul>
</dd></dl>
<dl class="data">
<dt id="typing.Union">
<code class="descclassname">typing.</code><code class="descname">Union</code><a class="headerlink" href="#typing.Union" title="Permalink to this definition">¶</a></dt>
<dd><p>Union type; <code class="docutils literal"><span class="pre">Union[X,</span> <span class="pre">Y]</span></code> means either X or Y.</p>
<p>To define a union, use e.g. <code class="docutils literal"><span class="pre">Union[int,</span> <span class="pre">str]</span></code>. Details:</p>
<ul>
<li><p class="first">The arguments must be types and there must be at least one.</p>
</li>
<li><p class="first">Unions of unions are flattened, e.g.:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">Union</span><span class="p">[</span><span class="n">Union</span><span class="p">[</span><span class="nb">int</span><span class="p">,</span> <span class="nb">str</span><span class="p">],</span> <span class="nb">float</span><span class="p">]</span> <span class="o">==</span> <span class="n">Union</span><span class="p">[</span><span class="nb">int</span><span class="p">,</span> <span class="nb">str</span><span class="p">,</span> <span class="nb">float</span><span class="p">]</span>
</pre></div>
</div>
</li>
<li><p class="first">Unions of a single argument vanish, e.g.:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">Union</span><span class="p">[</span><span class="nb">int</span><span class="p">]</span> <span class="o">==</span> <span class="nb">int</span> <span class="c1"># The constructor actually returns int</span>
</pre></div>
</div>
</li>
<li><p class="first">Redundant arguments are skipped, e.g.:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">Union</span><span class="p">[</span><span class="nb">int</span><span class="p">,</span> <span class="nb">str</span><span class="p">,</span> <span class="nb">int</span><span class="p">]</span> <span class="o">==</span> <span class="n">Union</span><span class="p">[</span><span class="nb">int</span><span class="p">,</span> <span class="nb">str</span><span class="p">]</span>
</pre></div>
</div>
</li>
<li><p class="first">When comparing unions, the argument order is ignored, e.g.:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">Union</span><span class="p">[</span><span class="nb">int</span><span class="p">,</span> <span class="nb">str</span><span class="p">]</span> <span class="o">==</span> <span class="n">Union</span><span class="p">[</span><span class="nb">str</span><span class="p">,</span> <span class="nb">int</span><span class="p">]</span>
</pre></div>
</div>
</li>
<li><p class="first">When a class and its subclass are present, the former is skipped, e.g.:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">Union</span><span class="p">[</span><span class="nb">int</span><span class="p">,</span> <span class="nb">object</span><span class="p">]</span> <span class="o">==</span> <span class="nb">object</span>
</pre></div>
</div>
</li>
<li><p class="first">You cannot subclass or instantiate a union.</p>
</li>
<li><p class="first">You cannot write <code class="docutils literal"><span class="pre">Union[X][Y]</span></code>.</p>
</li>
<li><p class="first">You can use <code class="docutils literal"><span class="pre">Optional[X]</span></code> as a shorthand for <code class="docutils literal"><span class="pre">Union[X,</span> <span class="pre">None]</span></code>.</p>
</li>
</ul>
</dd></dl>
<dl class="data">
<dt id="typing.Optional">
<code class="descclassname">typing.</code><code class="descname">Optional</code><a class="headerlink" href="#typing.Optional" title="Permalink to this definition">¶</a></dt>
<dd><p>Optional type.</p>
<p><code class="docutils literal"><span class="pre">Optional[X]</span></code> is equivalent to <code class="docutils literal"><span class="pre">Union[X,</span> <span class="pre">None]</span></code>.</p>
<p>Note that this is not the same concept as an optional argument,
which is one that has a default. An optional argument with a
default needn’t use the <code class="docutils literal"><span class="pre">Optional</span></code> qualifier on its type
annotation (although it is inferred if the default is <code class="docutils literal"><span class="pre">None</span></code>).
A mandatory argument may still have an <code class="docutils literal"><span class="pre">Optional</span></code> type if an
explicit value of <code class="docutils literal"><span class="pre">None</span></code> is allowed.</p>
</dd></dl>
<dl class="data">
<dt id="typing.Tuple">
<code class="descclassname">typing.</code><code class="descname">Tuple</code><a class="headerlink" href="#typing.Tuple" title="Permalink to this definition">¶</a></dt>
<dd><p>Tuple type; <code class="docutils literal"><span class="pre">Tuple[X,</span> <span class="pre">Y]</span></code> is the type of a tuple of two items
with the first item of type X and the second of type Y.</p>
<p>Example: <code class="docutils literal"><span class="pre">Tuple[T1,</span> <span class="pre">T2]</span></code> is a tuple of two elements corresponding
to type variables T1 and T2. <code class="docutils literal"><span class="pre">Tuple[int,</span> <span class="pre">float,</span> <span class="pre">str]</span></code> is a tuple
of an int, a float and a string.</p>
<p>To specify a variable-length tuple of homogeneous type,
use literal ellipsis, e.g. <code class="docutils literal"><span class="pre">Tuple[int,</span> <span class="pre">...]</span></code>. A plain <a class="reference internal" href="#typing.Tuple" title="typing.Tuple"><code class="xref py py-data docutils literal"><span class="pre">Tuple</span></code></a>
is equivalent to <code class="docutils literal"><span class="pre">Tuple[Any,</span> <span class="pre">...]</span></code>, and in turn to <a class="reference internal" href="stdtypes.html#tuple" title="tuple"><code class="xref py py-class docutils literal"><span class="pre">tuple</span></code></a>.</p>
</dd></dl>
<dl class="data">
<dt id="typing.Callable">
<code class="descclassname">typing.</code><code class="descname">Callable</code><a class="headerlink" href="#typing.Callable" title="Permalink to this definition">¶</a></dt>
<dd><p>Callable type; <code class="docutils literal"><span class="pre">Callable[[int],</span> <span class="pre">str]</span></code> is a function of (int) -> str.</p>
<p>The subscription syntax must always be used with exactly two
values: the argument list and the return type. The argument list
must be a list of types or an ellipsis; the return type must be
a single type.</p>
<p>There is no syntax to indicate optional or keyword arguments;
such function types are rarely used as callback types.
<code class="docutils literal"><span class="pre">Callable[...,</span> <span class="pre">ReturnType]</span></code> (literal ellipsis) can be used to
type hint a callable taking any number of arguments and returning
<code class="docutils literal"><span class="pre">ReturnType</span></code>. A plain <a class="reference internal" href="#typing.Callable" title="typing.Callable"><code class="xref py py-data docutils literal"><span class="pre">Callable</span></code></a> is equivalent to
<code class="docutils literal"><span class="pre">Callable[...,</span> <span class="pre">Any]</span></code>, and in turn to
<a class="reference internal" href="collections.abc.html#collections.abc.Callable" title="collections.abc.Callable"><code class="xref py py-class docutils literal"><span class="pre">collections.abc.Callable</span></code></a>.</p>
</dd></dl>
<dl class="data">
<dt id="typing.ClassVar">
<code class="descclassname">typing.</code><code class="descname">ClassVar</code><a class="headerlink" href="#typing.ClassVar" title="Permalink to this definition">¶</a></dt>
<dd><p>Special type construct to mark class variables.</p>
<p>As introduced in <span class="target" id="index-8"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0526"><strong>PEP 526</strong></a>, a variable annotation wrapped in ClassVar
indicates that a given attribute is intended to be used as a class variable
and should not be set on instances of that class. Usage:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Starship</span><span class="p">:</span>
<span class="n">stats</span> <span class="o">=</span> <span class="p">{}</span> <span class="c1"># type: ClassVar[Dict[str, int]] # class variable</span>
<span class="n">damage</span> <span class="o">=</span> <span class="mi">10</span> <span class="c1"># type: int # instance variable</span>
</pre></div>
</div>
<p><a class="reference internal" href="#typing.ClassVar" title="typing.ClassVar"><code class="xref py py-data docutils literal"><span class="pre">ClassVar</span></code></a> accepts only types and cannot be further subscribed.</p>
<p><a class="reference internal" href="#typing.ClassVar" title="typing.ClassVar"><code class="xref py py-data docutils literal"><span class="pre">ClassVar</span></code></a> is not a class itself, and should not
be used with <a class="reference internal" href="functions.html#isinstance" title="isinstance"><code class="xref py py-func docutils literal"><span class="pre">isinstance()</span></code></a> or <a class="reference internal" href="functions.html#issubclass" title="issubclass"><code class="xref py py-func docutils literal"><span class="pre">issubclass()</span></code></a>.
Note that <a class="reference internal" href="#typing.ClassVar" title="typing.ClassVar"><code class="xref py py-data docutils literal"><span class="pre">ClassVar</span></code></a> does not change Python runtime behavior;
it can be used by 3rd party type checkers, so that the following
code might flagged as an error by those:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">enterprise_d</span> <span class="o">=</span> <span class="n">Starship</span><span class="p">(</span><span class="mi">3000</span><span class="p">)</span>
<span class="n">enterprise_d</span><span class="o">.</span><span class="n">stats</span> <span class="o">=</span> <span class="p">{}</span> <span class="c1"># Error, setting class variable on instance</span>
<span class="n">Starship</span><span class="o">.</span><span class="n">stats</span> <span class="o">=</span> <span class="p">{}</span> <span class="c1"># This is OK</span>
</pre></div>
</div>
<div class="versionadded">
<p><span class="versionmodified">New in version 3.5.3.</span></p>
</div>
</dd></dl>
<dl class="data">
<dt id="typing.AnyStr">
<code class="descclassname">typing.</code><code class="descname">AnyStr</code><a class="headerlink" href="#typing.AnyStr" title="Permalink to this definition">¶</a></dt>
<dd><p><code class="docutils literal"><span class="pre">AnyStr</span></code> is a type variable defined as
<code class="docutils literal"><span class="pre">AnyStr</span> <span class="pre">=</span> <span class="pre">TypeVar('AnyStr',</span> <span class="pre">str,</span> <span class="pre">bytes)</span></code>.</p>
<p>It is meant to be used for functions that may accept any kind of string
without allowing different kinds of strings to mix. For example:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">concat</span><span class="p">(</span><span class="n">a</span><span class="p">:</span> <span class="n">AnyStr</span><span class="p">,</span> <span class="n">b</span><span class="p">:</span> <span class="n">AnyStr</span><span class="p">)</span> <span class="o">-></span> <span class="n">AnyStr</span><span class="p">:</span>
<span class="k">return</span> <span class="n">a</span> <span class="o">+</span> <span class="n">b</span>
<span class="n">concat</span><span class="p">(</span><span class="s2">u"foo"</span><span class="p">,</span> <span class="s2">u"bar"</span><span class="p">)</span> <span class="c1"># Ok, output has type 'unicode'</span>
<span class="n">concat</span><span class="p">(</span><span class="n">b</span><span class="s2">"foo"</span><span class="p">,</span> <span class="n">b</span><span class="s2">"bar"</span><span class="p">)</span> <span class="c1"># Ok, output has type 'bytes'</span>
<span class="n">concat</span><span class="p">(</span><span class="s2">u"foo"</span><span class="p">,</span> <span class="n">b</span><span class="s2">"bar"</span><span class="p">)</span> <span class="c1"># Error, cannot mix unicode and bytes</span>
</pre></div>
</div>
</dd></dl>
<dl class="data">
<dt id="typing.TYPE_CHECKING">
<code class="descclassname">typing.</code><code class="descname">TYPE_CHECKING</code><a class="headerlink" href="#typing.TYPE_CHECKING" title="Permalink to this definition">¶</a></dt>
<dd><p>A special constant that is assumed to be <code class="docutils literal"><span class="pre">True</span></code> by 3rd party static
type checkers. It is <code class="docutils literal"><span class="pre">False</span></code> at runtime. Usage:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="n">TYPE_CHECKING</span><span class="p">:</span>
<span class="kn">import</span> <span class="nn">expensive_mod</span>
<span class="k">def</span> <span class="nf">fun</span><span class="p">():</span>
<span class="n">local_var</span><span class="p">:</span> <span class="n">expensive_mod</span><span class="o">.</span><span class="n">some_type</span> <span class="o">=</span> <span class="n">other_fun</span><span class="p">()</span>
</pre></div>
</div>
</dd></dl>
</div>
</div>
</div>
</div>
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<h3><a href="../contents.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">26.1. <code class="docutils literal"><span class="pre">typing</span></code> — Support for type hints</a><ul>
<li><a class="reference internal" href="#type-aliases">26.1.1. Type aliases</a></li>
<li><a class="reference internal" href="#newtype">26.1.2. NewType</a></li>
<li><a class="reference internal" href="#callable">26.1.3. Callable</a></li>
<li><a class="reference internal" href="#generics">26.1.4. Generics</a></li>
<li><a class="reference internal" href="#user-defined-generic-types">26.1.5. User-defined generic types</a></li>
<li><a class="reference internal" href="#the-any-type">26.1.6. The <code class="docutils literal"><span class="pre">Any</span></code> type</a></li>
<li><a class="reference internal" href="#classes-functions-and-decorators">26.1.7. Classes, functions, and decorators</a></li>
</ul>
</li>
</ul>
<h4>Previous topic</h4>
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<h4>Next topic</h4>
<p class="topless"><a href="pydoc.html"
title="next chapter">26.2. <code class="docutils literal"><span class="pre">pydoc</span></code> — Documentation generator and online help system</a></p>
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