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<div class="section" id="execution-model">
<span id="execmodel"></span><h1><span class="section-number">4. </span>Execution model<a class="headerlink" href="#execution-model" title="Permalink to this headline">¶</a></h1>
<div class="section" id="structure-of-a-program">
<span id="prog-structure"></span><span id="index-0"></span><h2><span class="section-number">4.1. </span>Structure of a program<a class="headerlink" href="#structure-of-a-program" title="Permalink to this headline">¶</a></h2>
<p id="index-1">A Python program is constructed from code blocks.
A <em class="dfn">block</em> is a piece of Python program text that is executed as a unit.
The following are blocks: a module, a function body, and a class definition.
Each command typed interactively is a block. A script file (a file given as
standard input to the interpreter or specified as a command line argument to the
interpreter) is a code block. A script command (a command specified on the
interpreter command line with the <a class="reference internal" href="../using/cmdline.html#cmdoption-c"><code class="xref std std-option docutils literal notranslate"><span class="pre">-c</span></code></a> option) is a code block. The string
argument passed to the built-in functions <a class="reference internal" href="../library/functions.html#eval" title="eval"><code class="xref py py-func docutils literal notranslate"><span class="pre">eval()</span></code></a> and <a class="reference internal" href="../library/functions.html#exec" title="exec"><code class="xref py py-func docutils literal notranslate"><span class="pre">exec()</span></code></a> is a
code block.</p>
<p id="index-2">A code block is executed in an <em class="dfn">execution frame</em>. A frame contains some
administrative information (used for debugging) and determines where and how
execution continues after the code block’s execution has completed.</p>
</div>
<div class="section" id="naming-and-binding">
<span id="naming"></span><h2><span class="section-number">4.2. </span>Naming and binding<a class="headerlink" href="#naming-and-binding" title="Permalink to this headline">¶</a></h2>
<div class="section" id="binding-of-names">
<span id="bind-names"></span><span id="index-3"></span><h3><span class="section-number">4.2.1. </span>Binding of names<a class="headerlink" href="#binding-of-names" title="Permalink to this headline">¶</a></h3>
<p id="index-4"><em class="dfn">Names</em> refer to objects. Names are introduced by name binding operations.</p>
<p id="index-5">The following constructs bind names: formal parameters to functions,
<a class="reference internal" href="simple_stmts.html#import"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">import</span></code></a> statements, class and function definitions (these bind the
class or function name in the defining block), and targets that are identifiers
if occurring in an assignment, <a class="reference internal" href="compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> loop header, or after
<code class="xref std std-keyword docutils literal notranslate"><span class="pre">as</span></code> in a <a class="reference internal" href="compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement or <a class="reference internal" href="compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a> clause.
The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">import</span></code> statement
of the form <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">...</span> <span class="pre">import</span> <span class="pre">*</span></code> binds all names defined in the imported
module, except those beginning with an underscore. This form may only be used
at the module level.</p>
<p>A target occurring in a <a class="reference internal" href="simple_stmts.html#del"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">del</span></code></a> statement is also considered bound for
this purpose (though the actual semantics are to unbind the name).</p>
<p>Each assignment or import statement occurs within a block defined by a class or
function definition or at the module level (the top-level code block).</p>
<p id="index-6">If a name is bound in a block, it is a local variable of that block, unless
declared as <a class="reference internal" href="simple_stmts.html#nonlocal"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">nonlocal</span></code></a> or <a class="reference internal" href="simple_stmts.html#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a>. If a name is bound at
the module level, it is a global variable. (The variables of the module code
block are local and global.) If a variable is used in a code block but not
defined there, it is a <em class="dfn">free variable</em>.</p>
<p>Each occurrence of a name in the program text refers to the <em class="dfn">binding</em> of
that name established by the following name resolution rules.</p>
</div>
<div class="section" id="resolution-of-names">
<span id="resolve-names"></span><h3><span class="section-number">4.2.2. </span>Resolution of names<a class="headerlink" href="#resolution-of-names" title="Permalink to this headline">¶</a></h3>
<p id="index-7">A <em class="dfn">scope</em> defines the visibility of a name within a block. If a local
variable is defined in a block, its scope includes that block. If the
definition occurs in a function block, the scope extends to any blocks contained
within the defining one, unless a contained block introduces a different binding
for the name.</p>
<p id="index-8">When a name is used in a code block, it is resolved using the nearest enclosing
scope. The set of all such scopes visible to a code block is called the block’s
<em class="dfn">environment</em>.</p>
<p id="index-9">When a name is not found at all, a <a class="reference internal" href="../library/exceptions.html#NameError" title="NameError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">NameError</span></code></a> exception is raised.
If the current scope is a function scope, and the name refers to a local
variable that has not yet been bound to a value at the point where the name is
used, an <a class="reference internal" href="../library/exceptions.html#UnboundLocalError" title="UnboundLocalError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">UnboundLocalError</span></code></a> exception is raised.
<a class="reference internal" href="../library/exceptions.html#UnboundLocalError" title="UnboundLocalError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">UnboundLocalError</span></code></a> is a subclass of <a class="reference internal" href="../library/exceptions.html#NameError" title="NameError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">NameError</span></code></a>.</p>
<p>If a name binding operation occurs anywhere within a code block, all uses of the
name within the block are treated as references to the current block. This can
lead to errors when a name is used within a block before it is bound. This rule
is subtle. Python lacks declarations and allows name binding operations to
occur anywhere within a code block. The local variables of a code block can be
determined by scanning the entire text of the block for name binding operations.</p>
<p>If the <a class="reference internal" href="simple_stmts.html#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a> statement occurs within a block, all uses of the name
specified in the statement refer to the binding of that name in the top-level
namespace. Names are resolved in the top-level namespace by searching the
global namespace, i.e. the namespace of the module containing the code block,
and the builtins namespace, the namespace of the module <a class="reference internal" href="../library/builtins.html#module-builtins" title="builtins: The module that provides the built-in namespace."><code class="xref py py-mod docutils literal notranslate"><span class="pre">builtins</span></code></a>. The
global namespace is searched first. If the name is not found there, the
builtins namespace is searched. The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code> statement must precede
all uses of the name.</p>
<p>The <a class="reference internal" href="simple_stmts.html#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a> statement has the same scope as a name binding operation
in the same block. If the nearest enclosing scope for a free variable contains
a global statement, the free variable is treated as a global.</p>
<p>The <a class="reference internal" href="simple_stmts.html#nonlocal"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">nonlocal</span></code></a> statement causes corresponding names to refer
to previously bound variables in the nearest enclosing function scope.
<a class="reference internal" href="../library/exceptions.html#SyntaxError" title="SyntaxError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SyntaxError</span></code></a> is raised at compile time if the given name does not
exist in any enclosing function scope.</p>
<p id="index-10">The namespace for a module is automatically created the first time a module is
imported. The main module for a script is always called <a class="reference internal" href="../library/__main__.html#module-__main__" title="__main__: The environment where the top-level script is run."><code class="xref py py-mod docutils literal notranslate"><span class="pre">__main__</span></code></a>.</p>
<p>Class definition blocks and arguments to <a class="reference internal" href="../library/functions.html#exec" title="exec"><code class="xref py py-func docutils literal notranslate"><span class="pre">exec()</span></code></a> and <a class="reference internal" href="../library/functions.html#eval" title="eval"><code class="xref py py-func docutils literal notranslate"><span class="pre">eval()</span></code></a> are
special in the context of name resolution.
A class definition is an executable statement that may use and define names.
These references follow the normal rules for name resolution with an exception
that unbound local variables are looked up in the global namespace.
The namespace of the class definition becomes the attribute dictionary of
the class. The scope of names defined in a class block is limited to the
class block; it does not extend to the code blocks of methods – this includes
comprehensions and generator expressions since they are implemented using a
function scope. This means that the following will fail:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">A</span><span class="p">:</span>
<span class="n">a</span> <span class="o">=</span> <span class="mi">42</span>
<span class="n">b</span> <span class="o">=</span> <span class="nb">list</span><span class="p">(</span><span class="n">a</span> <span class="o">+</span> <span class="n">i</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">))</span>
</pre></div>
</div>
</div>
<div class="section" id="builtins-and-restricted-execution">
<span id="restrict-exec"></span><h3><span class="section-number">4.2.3. </span>Builtins and restricted execution<a class="headerlink" href="#builtins-and-restricted-execution" title="Permalink to this headline">¶</a></h3>
<div class="impl-detail compound" id="index-11">
<p><strong>CPython implementation detail:</strong> Users should not touch <code class="docutils literal notranslate"><span class="pre">__builtins__</span></code>; it is strictly an implementation
detail. Users wanting to override values in the builtins namespace should
<a class="reference internal" href="simple_stmts.html#import"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">import</span></code></a> the <a class="reference internal" href="../library/builtins.html#module-builtins" title="builtins: The module that provides the built-in namespace."><code class="xref py py-mod docutils literal notranslate"><span class="pre">builtins</span></code></a> module and modify its
attributes appropriately.</p>
</div>
<p>The builtins namespace associated with the execution of a code block
is actually found by looking up the name <code class="docutils literal notranslate"><span class="pre">__builtins__</span></code> in its
global namespace; this should be a dictionary or a module (in the
latter case the module’s dictionary is used). By default, when in the
<a class="reference internal" href="../library/__main__.html#module-__main__" title="__main__: The environment where the top-level script is run."><code class="xref py py-mod docutils literal notranslate"><span class="pre">__main__</span></code></a> module, <code class="docutils literal notranslate"><span class="pre">__builtins__</span></code> is the built-in module
<a class="reference internal" href="../library/builtins.html#module-builtins" title="builtins: The module that provides the built-in namespace."><code class="xref py py-mod docutils literal notranslate"><span class="pre">builtins</span></code></a>; when in any other module, <code class="docutils literal notranslate"><span class="pre">__builtins__</span></code> is an
alias for the dictionary of the <a class="reference internal" href="../library/builtins.html#module-builtins" title="builtins: The module that provides the built-in namespace."><code class="xref py py-mod docutils literal notranslate"><span class="pre">builtins</span></code></a> module itself.</p>
</div>
<div class="section" id="interaction-with-dynamic-features">
<span id="dynamic-features"></span><h3><span class="section-number">4.2.4. </span>Interaction with dynamic features<a class="headerlink" href="#interaction-with-dynamic-features" title="Permalink to this headline">¶</a></h3>
<p>Name resolution of free variables occurs at runtime, not at compile time.
This means that the following code will print 42:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">i</span> <span class="o">=</span> <span class="mi">10</span>
<span class="k">def</span> <span class="nf">f</span><span class="p">():</span>
<span class="nb">print</span><span class="p">(</span><span class="n">i</span><span class="p">)</span>
<span class="n">i</span> <span class="o">=</span> <span class="mi">42</span>
<span class="n">f</span><span class="p">()</span>
</pre></div>
</div>
<p>The <a class="reference internal" href="../library/functions.html#eval" title="eval"><code class="xref py py-func docutils literal notranslate"><span class="pre">eval()</span></code></a> and <a class="reference internal" href="../library/functions.html#exec" title="exec"><code class="xref py py-func docutils literal notranslate"><span class="pre">exec()</span></code></a> functions do not have access to the full
environment for resolving names. Names may be resolved in the local and global
namespaces of the caller. Free variables are not resolved in the nearest
enclosing namespace, but in the global namespace. <a class="footnote-reference brackets" href="#id3" id="id1">1</a> The <a class="reference internal" href="../library/functions.html#exec" title="exec"><code class="xref py py-func docutils literal notranslate"><span class="pre">exec()</span></code></a> and
<a class="reference internal" href="../library/functions.html#eval" title="eval"><code class="xref py py-func docutils literal notranslate"><span class="pre">eval()</span></code></a> functions have optional arguments to override the global and local
namespace. If only one namespace is specified, it is used for both.</p>
</div>
</div>
<div class="section" id="exceptions">
<span id="id2"></span><h2><span class="section-number">4.3. </span>Exceptions<a class="headerlink" href="#exceptions" title="Permalink to this headline">¶</a></h2>
<span class="target" id="index-12"></span><p id="index-13">Exceptions are a means of breaking out of the normal flow of control of a code
block in order to handle errors or other exceptional conditions. An exception
is <em>raised</em> at the point where the error is detected; it may be <em>handled</em> by the
surrounding code block or by any code block that directly or indirectly invoked
the code block where the error occurred.</p>
<p>The Python interpreter raises an exception when it detects a run-time error
(such as division by zero). A Python program can also explicitly raise an
exception with the <a class="reference internal" href="simple_stmts.html#raise"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">raise</span></code></a> statement. Exception handlers are specified
with the <a class="reference internal" href="compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> … <a class="reference internal" href="compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a> statement. The <a class="reference internal" href="compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a>
clause of such a statement can be used to specify cleanup code which does not
handle the exception, but is executed whether an exception occurred or not in
the preceding code.</p>
<p id="index-14">Python uses the “termination” model of error handling: an exception handler can
find out what happened and continue execution at an outer level, but it cannot
repair the cause of the error and retry the failing operation (except by
re-entering the offending piece of code from the top).</p>
<p id="index-15">When an exception is not handled at all, the interpreter terminates execution of
the program, or returns to its interactive main loop. In either case, it prints
a stack traceback, except when the exception is <a class="reference internal" href="../library/exceptions.html#SystemExit" title="SystemExit"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SystemExit</span></code></a>.</p>
<p>Exceptions are identified by class instances. The <a class="reference internal" href="compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a> clause is
selected depending on the class of the instance: it must reference the class of
the instance or a base class thereof. The instance can be received by the
handler and can carry additional information about the exceptional condition.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>Exception messages are not part of the Python API. Their contents may change
from one version of Python to the next without warning and should not be
relied on by code which will run under multiple versions of the interpreter.</p>
</div>
<p>See also the description of the <a class="reference internal" href="compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> statement in section <a class="reference internal" href="compound_stmts.html#try"><span class="std std-ref">The try statement</span></a>
and <a class="reference internal" href="simple_stmts.html#raise"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">raise</span></code></a> statement in section <a class="reference internal" href="simple_stmts.html#raise"><span class="std std-ref">The raise statement</span></a>.</p>
<p class="rubric">Footnotes</p>
<dl class="footnote brackets">
<dt class="label" id="id3"><span class="brackets"><a class="fn-backref" href="#id1">1</a></span></dt>
<dd><p>This limitation occurs because the code that is executed by these operations
is not available at the time the module is compiled.</p>
</dd>
</dl>
</div>
</div>
</div>
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<h3><a href="../contents.html">Table of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">4. Execution model</a><ul>
<li><a class="reference internal" href="#structure-of-a-program">4.1. Structure of a program</a></li>
<li><a class="reference internal" href="#naming-and-binding">4.2. Naming and binding</a><ul>
<li><a class="reference internal" href="#binding-of-names">4.2.1. Binding of names</a></li>
<li><a class="reference internal" href="#resolution-of-names">4.2.2. Resolution of names</a></li>
<li><a class="reference internal" href="#builtins-and-restricted-execution">4.2.3. Builtins and restricted execution</a></li>
<li><a class="reference internal" href="#interaction-with-dynamic-features">4.2.4. Interaction with dynamic features</a></li>
</ul>
</li>
<li><a class="reference internal" href="#exceptions">4.3. Exceptions</a></li>
</ul>
</li>
</ul>
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