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  <div class="section" id="module-functools">
<span id="functools-higher-order-functions-and-operations-on-callable-objects"></span><h1>9.8. <a class="reference internal" href="#module-functools" title="functools: Higher-order functions and operations on callable objects."><code class="xref py py-mod docutils literal notranslate"><span class="pre">functools</span></code></a> — Higher-order functions and operations on callable objects<a class="headerlink" href="#module-functools" title="Permalink to this headline">¶</a></h1>
<div class="versionadded">
<p><span class="versionmodified added">New in version 2.5.</span></p>
</div>
<p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/2.7/Lib/functools.py">Lib/functools.py</a></p>
<hr class="docutils" />
<p>The <a class="reference internal" href="#module-functools" title="functools: Higher-order functions and operations on callable objects."><code class="xref py py-mod docutils literal notranslate"><span class="pre">functools</span></code></a> module is for higher-order functions: functions that act on
or return other functions. In general, any callable object can be treated as a
function for the purposes of this module.</p>
<p>The <a class="reference internal" href="#module-functools" title="functools: Higher-order functions and operations on callable objects."><code class="xref py py-mod docutils literal notranslate"><span class="pre">functools</span></code></a> module defines the following functions:</p>
<dl class="function">
<dt id="functools.cmp_to_key">
<code class="descclassname">functools.</code><code class="descname">cmp_to_key</code><span class="sig-paren">(</span><em>func</em><span class="sig-paren">)</span><a class="headerlink" href="#functools.cmp_to_key" title="Permalink to this definition">¶</a></dt>
<dd><p>Transform an old-style comparison function to a <a class="reference internal" href="../glossary.html#term-key-function"><span class="xref std std-term">key function</span></a>.  Used
with tools that accept key functions (such as <a class="reference internal" href="functions.html#sorted" title="sorted"><code class="xref py py-func docutils literal notranslate"><span class="pre">sorted()</span></code></a>, <a class="reference internal" href="functions.html#min" title="min"><code class="xref py py-func docutils literal notranslate"><span class="pre">min()</span></code></a>,
<a class="reference internal" href="functions.html#max" title="max"><code class="xref py py-func docutils literal notranslate"><span class="pre">max()</span></code></a>, <a class="reference internal" href="heapq.html#heapq.nlargest" title="heapq.nlargest"><code class="xref py py-func docutils literal notranslate"><span class="pre">heapq.nlargest()</span></code></a>, <a class="reference internal" href="heapq.html#heapq.nsmallest" title="heapq.nsmallest"><code class="xref py py-func docutils literal notranslate"><span class="pre">heapq.nsmallest()</span></code></a>,
<a class="reference internal" href="itertools.html#itertools.groupby" title="itertools.groupby"><code class="xref py py-func docutils literal notranslate"><span class="pre">itertools.groupby()</span></code></a>).  This function is primarily used as a transition
tool for programs being converted to Python 3 where comparison functions are
no longer supported.</p>
<p>A comparison function is any callable that accept two arguments, compares them,
and returns a negative number for less-than, zero for equality, or a positive
number for greater-than.  A key function is a callable that accepts one
argument and returns another value to be used as the sort key.</p>
<p>Example:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="nb">sorted</span><span class="p">(</span><span class="n">iterable</span><span class="p">,</span> <span class="n">key</span><span class="o">=</span><span class="n">cmp_to_key</span><span class="p">(</span><span class="n">locale</span><span class="o">.</span><span class="n">strcoll</span><span class="p">))</span>  <span class="c1"># locale-aware sort order</span>
</pre></div>
</div>
<p>For sorting examples and a brief sorting tutorial, see <a class="reference internal" href="../howto/sorting.html#sortinghowto"><span class="std std-ref">Sorting HOW TO</span></a>.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 2.7.</span></p>
</div>
</dd></dl>

<dl class="function">
<dt id="functools.total_ordering">
<code class="descclassname">functools.</code><code class="descname">total_ordering</code><span class="sig-paren">(</span><em>cls</em><span class="sig-paren">)</span><a class="headerlink" href="#functools.total_ordering" title="Permalink to this definition">¶</a></dt>
<dd><p>Given a class defining one or more rich comparison ordering methods, this
class decorator supplies the rest.  This simplifies the effort involved
in specifying all of the possible rich comparison operations:</p>
<p>The class must define one of <a class="reference internal" href="../reference/datamodel.html#object.__lt__" title="object.__lt__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__lt__()</span></code></a>, <a class="reference internal" href="../reference/datamodel.html#object.__le__" title="object.__le__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__le__()</span></code></a>,
<a class="reference internal" href="../reference/datamodel.html#object.__gt__" title="object.__gt__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__gt__()</span></code></a>, or <a class="reference internal" href="../reference/datamodel.html#object.__ge__" title="object.__ge__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__ge__()</span></code></a>.
In addition, the class should supply an <a class="reference internal" href="../reference/datamodel.html#object.__eq__" title="object.__eq__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__eq__()</span></code></a> method.</p>
<p>For example:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="nd">@total_ordering</span>
<span class="k">class</span> <span class="nc">Student</span><span class="p">:</span>
    <span class="k">def</span> <span class="nf">__eq__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">other</span><span class="p">):</span>
        <span class="k">return</span> <span class="p">((</span><span class="bp">self</span><span class="o">.</span><span class="n">lastname</span><span class="o">.</span><span class="n">lower</span><span class="p">(),</span> <span class="bp">self</span><span class="o">.</span><span class="n">firstname</span><span class="o">.</span><span class="n">lower</span><span class="p">())</span> <span class="o">==</span>
                <span class="p">(</span><span class="n">other</span><span class="o">.</span><span class="n">lastname</span><span class="o">.</span><span class="n">lower</span><span class="p">(),</span> <span class="n">other</span><span class="o">.</span><span class="n">firstname</span><span class="o">.</span><span class="n">lower</span><span class="p">()))</span>
    <span class="k">def</span> <span class="nf">__lt__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">other</span><span class="p">):</span>
        <span class="k">return</span> <span class="p">((</span><span class="bp">self</span><span class="o">.</span><span class="n">lastname</span><span class="o">.</span><span class="n">lower</span><span class="p">(),</span> <span class="bp">self</span><span class="o">.</span><span class="n">firstname</span><span class="o">.</span><span class="n">lower</span><span class="p">())</span> <span class="o">&lt;</span>
                <span class="p">(</span><span class="n">other</span><span class="o">.</span><span class="n">lastname</span><span class="o">.</span><span class="n">lower</span><span class="p">(),</span> <span class="n">other</span><span class="o">.</span><span class="n">firstname</span><span class="o">.</span><span class="n">lower</span><span class="p">()))</span>
</pre></div>
</div>
<div class="versionadded">
<p><span class="versionmodified added">New in version 2.7.</span></p>
</div>
</dd></dl>

<dl class="function">
<dt id="functools.reduce">
<code class="descclassname">functools.</code><code class="descname">reduce</code><span class="sig-paren">(</span><em>function</em>, <em>iterable</em><span class="optional">[</span>, <em>initializer</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#functools.reduce" title="Permalink to this definition">¶</a></dt>
<dd><p>This is the same function as <a class="reference internal" href="functions.html#reduce" title="reduce"><code class="xref py py-func docutils literal notranslate"><span class="pre">reduce()</span></code></a>.  It is made available in this module
to allow writing code more forward-compatible with Python 3.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 2.6.</span></p>
</div>
</dd></dl>

<dl class="function">
<dt id="functools.partial">
<code class="descclassname">functools.</code><code class="descname">partial</code><span class="sig-paren">(</span><em>func[,*args][, **keywords]</em><span class="sig-paren">)</span><a class="headerlink" href="#functools.partial" title="Permalink to this definition">¶</a></dt>
<dd><p>Return a new <a class="reference internal" href="#partial-objects"><span class="std std-ref">partial object</span></a> which when called will behave
like <em>func</em> called with the positional arguments <em>args</em> and keyword arguments <em>keywords</em>.
If more arguments are supplied to the call, they are appended to <em>args</em>. If
additional keyword arguments are supplied, they extend and override <em>keywords</em>.
Roughly equivalent to:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">partial</span><span class="p">(</span><span class="n">func</span><span class="p">,</span> <span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">keywords</span><span class="p">):</span>
    <span class="k">def</span> <span class="nf">newfunc</span><span class="p">(</span><span class="o">*</span><span class="n">fargs</span><span class="p">,</span> <span class="o">**</span><span class="n">fkeywords</span><span class="p">):</span>
        <span class="n">newkeywords</span> <span class="o">=</span> <span class="n">keywords</span><span class="o">.</span><span class="n">copy</span><span class="p">()</span>
        <span class="n">newkeywords</span><span class="o">.</span><span class="n">update</span><span class="p">(</span><span class="n">fkeywords</span><span class="p">)</span>
        <span class="k">return</span> <span class="n">func</span><span class="p">(</span><span class="o">*</span><span class="p">(</span><span class="n">args</span> <span class="o">+</span> <span class="n">fargs</span><span class="p">),</span> <span class="o">**</span><span class="n">newkeywords</span><span class="p">)</span>
    <span class="n">newfunc</span><span class="o">.</span><span class="n">func</span> <span class="o">=</span> <span class="n">func</span>
    <span class="n">newfunc</span><span class="o">.</span><span class="n">args</span> <span class="o">=</span> <span class="n">args</span>
    <span class="n">newfunc</span><span class="o">.</span><span class="n">keywords</span> <span class="o">=</span> <span class="n">keywords</span>
    <span class="k">return</span> <span class="n">newfunc</span>
</pre></div>
</div>
<p>The <a class="reference internal" href="#functools.partial" title="functools.partial"><code class="xref py py-func docutils literal notranslate"><span class="pre">partial()</span></code></a> is used for partial function application which “freezes”
some portion of a function’s arguments and/or keywords resulting in a new object
with a simplified signature.  For example, <a class="reference internal" href="#functools.partial" title="functools.partial"><code class="xref py py-func docutils literal notranslate"><span class="pre">partial()</span></code></a> can be used to create
a callable that behaves like the <a class="reference internal" href="functions.html#int" title="int"><code class="xref py py-func docutils literal notranslate"><span class="pre">int()</span></code></a> function where the <em>base</em> argument
defaults to two:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="kn">from</span> <span class="nn">functools</span> <span class="k">import</span> <span class="n">partial</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">basetwo</span> <span class="o">=</span> <span class="n">partial</span><span class="p">(</span><span class="nb">int</span><span class="p">,</span> <span class="n">base</span><span class="o">=</span><span class="mi">2</span><span class="p">)</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">basetwo</span><span class="o">.</span><span class="vm">__doc__</span> <span class="o">=</span> <span class="s1">&#39;Convert base 2 string to an int.&#39;</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">basetwo</span><span class="p">(</span><span class="s1">&#39;10010&#39;</span><span class="p">)</span>
<span class="go">18</span>
</pre></div>
</div>
</dd></dl>

<dl class="function">
<dt id="functools.update_wrapper">
<code class="descclassname">functools.</code><code class="descname">update_wrapper</code><span class="sig-paren">(</span><em>wrapper, wrapped[, assigned][, updated]</em><span class="sig-paren">)</span><a class="headerlink" href="#functools.update_wrapper" title="Permalink to this definition">¶</a></dt>
<dd><p>Update a <em>wrapper</em> function to look like the <em>wrapped</em> function. The optional
arguments are tuples to specify which attributes of the original function are
assigned directly to the matching attributes on the wrapper function and which
attributes of the wrapper function are updated with the corresponding attributes
from the original function. The default values for these arguments are the
module level constants <em>WRAPPER_ASSIGNMENTS</em> (which assigns to the wrapper
function’s <em>__name__</em>, <em>__module__</em> and <em>__doc__</em>, the documentation string) and
<em>WRAPPER_UPDATES</em> (which updates the wrapper function’s <em>__dict__</em>, i.e. the
instance dictionary).</p>
<p>The main intended use for this function is in <a class="reference internal" href="../glossary.html#term-decorator"><span class="xref std std-term">decorator</span></a> functions which
wrap the decorated function and return the wrapper. If the wrapper function is
not updated, the metadata of the returned function will reflect the wrapper
definition rather than the original function definition, which is typically less
than helpful.</p>
</dd></dl>

<dl class="function">
<dt id="functools.wraps">
<code class="descclassname">functools.</code><code class="descname">wraps</code><span class="sig-paren">(</span><em>wrapped[, assigned][, updated]</em><span class="sig-paren">)</span><a class="headerlink" href="#functools.wraps" title="Permalink to this definition">¶</a></dt>
<dd><p>This is a convenience function for invoking <a class="reference internal" href="#functools.update_wrapper" title="functools.update_wrapper"><code class="xref py py-func docutils literal notranslate"><span class="pre">update_wrapper()</span></code></a> as a
function decorator when defining a wrapper function.  It is equivalent to
<code class="docutils literal notranslate"><span class="pre">partial(update_wrapper,</span> <span class="pre">wrapped=wrapped,</span> <span class="pre">assigned=assigned,</span> <span class="pre">updated=updated)</span></code>.
For example:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="kn">from</span> <span class="nn">functools</span> <span class="k">import</span> <span class="n">wraps</span>
<span class="gp">&gt;&gt;&gt; </span><span class="k">def</span> <span class="nf">my_decorator</span><span class="p">(</span><span class="n">f</span><span class="p">):</span>
<span class="gp">... </span>    <span class="nd">@wraps</span><span class="p">(</span><span class="n">f</span><span class="p">)</span>
<span class="gp">... </span>    <span class="k">def</span> <span class="nf">wrapper</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwds</span><span class="p">):</span>
<span class="gp">... </span>        <span class="nb">print</span> <span class="s1">&#39;Calling decorated function&#39;</span>
<span class="gp">... </span>        <span class="k">return</span> <span class="n">f</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwds</span><span class="p">)</span>
<span class="gp">... </span>    <span class="k">return</span> <span class="n">wrapper</span>
<span class="gp">...</span>
<span class="gp">&gt;&gt;&gt; </span><span class="nd">@my_decorator</span>
<span class="gp">... </span><span class="k">def</span> <span class="nf">example</span><span class="p">():</span>
<span class="gp">... </span>    <span class="sd">&quot;&quot;&quot;Docstring&quot;&quot;&quot;</span>
<span class="gp">... </span>    <span class="nb">print</span> <span class="s1">&#39;Called example function&#39;</span>
<span class="gp">...</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">example</span><span class="p">()</span>
<span class="go">Calling decorated function</span>
<span class="go">Called example function</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">example</span><span class="o">.</span><span class="vm">__name__</span>
<span class="go">&#39;example&#39;</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">example</span><span class="o">.</span><span class="vm">__doc__</span>
<span class="go">&#39;Docstring&#39;</span>
</pre></div>
</div>
<p>Without the use of this decorator factory, the name of the example function
would have been <code class="docutils literal notranslate"><span class="pre">'wrapper'</span></code>, and the docstring of the original <code class="xref py py-func docutils literal notranslate"><span class="pre">example()</span></code>
would have been lost.</p>
</dd></dl>

<div class="section" id="partial-objects">
<span id="id1"></span><h2>9.8.1. <a class="reference internal" href="#functools.partial" title="functools.partial"><code class="xref py py-class docutils literal notranslate"><span class="pre">partial</span></code></a> Objects<a class="headerlink" href="#partial-objects" title="Permalink to this headline">¶</a></h2>
<p><a class="reference internal" href="#functools.partial" title="functools.partial"><code class="xref py py-class docutils literal notranslate"><span class="pre">partial</span></code></a> objects are callable objects created by <a class="reference internal" href="#functools.partial" title="functools.partial"><code class="xref py py-func docutils literal notranslate"><span class="pre">partial()</span></code></a>. They
have three read-only attributes:</p>
<dl class="attribute">
<dt id="functools.partial.func">
<code class="descclassname">partial.</code><code class="descname">func</code><a class="headerlink" href="#functools.partial.func" title="Permalink to this definition">¶</a></dt>
<dd><p>A callable object or function.  Calls to the <a class="reference internal" href="#functools.partial" title="functools.partial"><code class="xref py py-class docutils literal notranslate"><span class="pre">partial</span></code></a> object will be
forwarded to <a class="reference internal" href="#functools.partial.func" title="functools.partial.func"><code class="xref py py-attr docutils literal notranslate"><span class="pre">func</span></code></a> with new arguments and keywords.</p>
</dd></dl>

<dl class="attribute">
<dt id="functools.partial.args">
<code class="descclassname">partial.</code><code class="descname">args</code><a class="headerlink" href="#functools.partial.args" title="Permalink to this definition">¶</a></dt>
<dd><p>The leftmost positional arguments that will be prepended to the positional
arguments provided to a <a class="reference internal" href="#functools.partial" title="functools.partial"><code class="xref py py-class docutils literal notranslate"><span class="pre">partial</span></code></a> object call.</p>
</dd></dl>

<dl class="attribute">
<dt id="functools.partial.keywords">
<code class="descclassname">partial.</code><code class="descname">keywords</code><a class="headerlink" href="#functools.partial.keywords" title="Permalink to this definition">¶</a></dt>
<dd><p>The keyword arguments that will be supplied when the <a class="reference internal" href="#functools.partial" title="functools.partial"><code class="xref py py-class docutils literal notranslate"><span class="pre">partial</span></code></a> object is
called.</p>
</dd></dl>

<p><a class="reference internal" href="#functools.partial" title="functools.partial"><code class="xref py py-class docutils literal notranslate"><span class="pre">partial</span></code></a> objects are like <code class="xref py py-class docutils literal notranslate"><span class="pre">function</span></code> objects in that they are
callable, weak referencable, and can have attributes.  There are some important
differences.  For instance, the <a class="reference internal" href="stdtypes.html#definition.__name__" title="definition.__name__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__name__</span></code></a> and <code class="xref py py-attr docutils literal notranslate"><span class="pre">__doc__</span></code> attributes
are not created automatically.  Also, <a class="reference internal" href="#functools.partial" title="functools.partial"><code class="xref py py-class docutils literal notranslate"><span class="pre">partial</span></code></a> objects defined in
classes behave like static methods and do not transform into bound methods
during instance attribute look-up.</p>
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