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<div class="section" id="execution-model">
<span id="execmodel"></span><h1>4. Execution model<a class="headerlink" href="#execution-model" title="Permalink to this headline">¶</a></h1>
<div class="section" id="naming-and-binding">
<span id="naming"></span><span id="index-916"></span><h2>4.1. Naming and binding<a class="headerlink" href="#naming-and-binding" title="Permalink to this headline">¶</a></h2>
<span class="target" id="index-917"></span><p id="index-918"><em>Names</em> refer to objects. Names are introduced by name binding operations.
Each occurrence of a name in the program text refers to the <em>binding</em> of
that name established in the innermost function block containing the use.</p>
<p id="index-919">A <em>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 on the interpreter command line
the first argument) is a code block. A script command (a command specified on
the interpreter command line with the ‘<strong>-c</strong>‘ option) is a code block. The
file read by the built-in function <a title="execfile" class="reference external" href="../library/functions.html#execfile"><tt class="xref docutils literal"><span class="pre">execfile()</span></tt></a> is a code block. The string
argument passed to the built-in function <a title="eval" class="reference external" href="../library/functions.html#eval"><tt class="xref docutils literal"><span class="pre">eval()</span></tt></a> and to the <a class="reference external" href="simple_stmts.html#exec"><tt class="xref docutils literal"><span class="pre">exec</span></tt></a>
statement is a code block. The expression read and evaluated by the built-in
function <a title="input" class="reference external" href="../library/functions.html#input"><tt class="xref docutils literal"><span class="pre">input()</span></tt></a> is a code block.</p>
<p id="index-920">A code block is executed in an <em>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>
<p id="index-921">A <em>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. 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
generator expressions since they are implemented using a function scope. This
means that the following will fail:</p>
<div class="highlight-python"><div class="highlight"><pre><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>
<p id="index-922">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>environment</em>.</p>
<p id="index-923">If a name is bound in a block, it is a local variable of that block. 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>free variable</em>.</p>
<p id="index-924">When a name is not found at all, a <a title="exceptions.NameError" class="reference external" href="../library/exceptions.html#exceptions.NameError"><tt class="xref docutils literal"><span class="pre">NameError</span></tt></a> exception is raised. If the
name refers to a local variable that has not been bound, a
<a title="exceptions.UnboundLocalError" class="reference external" href="../library/exceptions.html#exceptions.UnboundLocalError"><tt class="xref docutils literal"><span class="pre">UnboundLocalError</span></tt></a> exception is raised. <a title="exceptions.UnboundLocalError" class="reference external" href="../library/exceptions.html#exceptions.UnboundLocalError"><tt class="xref docutils literal"><span class="pre">UnboundLocalError</span></tt></a> is a
subclass of <a title="exceptions.NameError" class="reference external" href="../library/exceptions.html#exceptions.NameError"><tt class="xref docutils literal"><span class="pre">NameError</span></tt></a>.</p>
<p id="index-925">The following constructs bind names: formal parameters to functions,
<a class="reference external" href="simple_stmts.html#import"><tt class="xref docutils literal"><span class="pre">import</span></tt></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 external" href="compound_stmts.html#for"><tt class="xref docutils literal"><span class="pre">for</span></tt></a> loop header, in the second
position of an <a class="reference external" href="compound_stmts.html#except"><tt class="xref docutils literal"><span class="pre">except</span></tt></a> clause header or after <a class="reference external" href="compound_stmts.html#as"><tt class="xref docutils literal"><span class="pre">as</span></tt></a> in a
<a class="reference external" href="compound_stmts.html#with"><tt class="xref docutils literal"><span class="pre">with</span></tt></a> statement. The <a class="reference external" href="simple_stmts.html#import"><tt class="xref docutils literal"><span class="pre">import</span></tt></a> statement
of the form <tt class="docutils literal"><span class="pre">from</span> <span class="pre">...</span> <span class="pre">import</span> <span class="pre">*</span></tt> 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 external" href="simple_stmts.html#del"><tt class="xref docutils literal"><span class="pre">del</span></tt></a> statement is also considered bound for
this purpose (though the actual semantics are to unbind the name). It is
illegal to unbind a name that is referenced by an enclosing scope; the compiler
will report a <a title="exceptions.SyntaxError" class="reference external" href="../library/exceptions.html#exceptions.SyntaxError"><tt class="xref docutils literal"><span class="pre">SyntaxError</span></tt></a>.</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>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 global 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 title="The module that provides the built-in namespace." class="reference external" href="../library/__builtin__.html#module-__builtin__"><tt class="xref docutils literal"><span class="pre">__builtin__</span></tt></a>. The global namespace
is searched first. If the name is not found there, the builtins namespace is
searched. The global statement must precede all uses of the name.</p>
<p id="index-926">The builtins namespace associated with the execution of a code block is actually
found by looking up the name <tt class="docutils literal"><span class="pre">__builtins__</span></tt> 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 title="The environment where the top-level script is run." class="reference external" href="../library/__main__.html#module-__main__"><tt class="xref docutils literal"><span class="pre">__main__</span></tt></a> module, <tt class="docutils literal"><span class="pre">__builtins__</span></tt> is
the built-in module <a title="The module that provides the built-in namespace." class="reference external" href="../library/__builtin__.html#module-__builtin__"><tt class="xref docutils literal"><span class="pre">__builtin__</span></tt></a> (note: no ‘s’); when in any other module,
<tt class="docutils literal"><span class="pre">__builtins__</span></tt> is an alias for the dictionary of the <a title="The module that provides the built-in namespace." class="reference external" href="../library/__builtin__.html#module-__builtin__"><tt class="xref docutils literal"><span class="pre">__builtin__</span></tt></a> module
itself. <tt class="docutils literal"><span class="pre">__builtins__</span></tt> can be set to a user-created dictionary to create a
weak form of restricted execution.</p>
<div class="impl-detail compound">
<p><strong>CPython implementation detail:</strong> Users should not touch <tt class="docutils literal"><span class="pre">__builtins__</span></tt>; it is strictly an implementation
detail. Users wanting to override values in the builtins namespace should
<a class="reference external" href="simple_stmts.html#import"><tt class="xref docutils literal"><span class="pre">import</span></tt></a> the <a title="The module that provides the built-in namespace." class="reference external" href="../library/__builtin__.html#module-__builtin__"><tt class="xref docutils literal"><span class="pre">__builtin__</span></tt></a> (no ‘s’) module and modify its
attributes appropriately.</p>
</div>
<p id="index-927">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 title="The environment where the top-level script is run." class="reference external" href="../library/__main__.html#module-__main__"><tt class="xref docutils literal"><span class="pre">__main__</span></tt></a>.</p>
<p>The global 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>A class definition is an executable statement that may use and define names.
These references follow the normal rules for name resolution. The namespace of
the class definition becomes the attribute dictionary of the class. Names
defined at the class scope are not visible in methods.</p>
<div class="section" id="interaction-with-dynamic-features">
<span id="dynamic-features"></span><h3>4.1.1. Interaction with dynamic features<a class="headerlink" href="#interaction-with-dynamic-features" title="Permalink to this headline">¶</a></h3>
<p>There are several cases where Python statements are illegal when used in
conjunction with nested scopes that contain free variables.</p>
<p>If a variable is referenced in an enclosing scope, it is illegal to delete the
name. An error will be reported at compile time.</p>
<p>If the wild card form of import — <tt class="docutils literal"><span class="pre">import</span> <span class="pre">*</span></tt> — is used in a function and
the function contains or is a nested block with free variables, the compiler
will raise a <a title="exceptions.SyntaxError" class="reference external" href="../library/exceptions.html#exceptions.SyntaxError"><tt class="xref docutils literal"><span class="pre">SyntaxError</span></tt></a>.</p>
<p>If <a class="reference external" href="simple_stmts.html#exec"><tt class="xref docutils literal"><span class="pre">exec</span></tt></a> is used in a function and the function contains or is a
nested block with free variables, the compiler will raise a <a title="exceptions.SyntaxError" class="reference external" href="../library/exceptions.html#exceptions.SyntaxError"><tt class="xref docutils literal"><span class="pre">SyntaxError</span></tt></a>
unless the exec explicitly specifies the local namespace for the
<a class="reference external" href="simple_stmts.html#exec"><tt class="xref docutils literal"><span class="pre">exec</span></tt></a>. (In other words, <tt class="docutils literal"><span class="pre">exec</span> <span class="pre">obj</span></tt> would be illegal, but <tt class="docutils literal"><span class="pre">exec</span> <span class="pre">obj</span>
<span class="pre">in</span> <span class="pre">ns</span></tt> would be legal.)</p>
<p>The <a title="eval" class="reference external" href="../library/functions.html#eval"><tt class="xref docutils literal"><span class="pre">eval()</span></tt></a>, <a title="execfile" class="reference external" href="../library/functions.html#execfile"><tt class="xref docutils literal"><span class="pre">execfile()</span></tt></a>, and <a title="input" class="reference external" href="../library/functions.html#input"><tt class="xref docutils literal"><span class="pre">input()</span></tt></a> functions and the
<a class="reference external" href="simple_stmts.html#exec"><tt class="xref docutils literal"><span class="pre">exec</span></tt></a> statement 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" href="#id3" id="id1">[1]</a> The <a class="reference external" href="simple_stmts.html#exec"><tt class="xref docutils literal"><span class="pre">exec</span></tt></a> statement and the
<a title="eval" class="reference external" href="../library/functions.html#eval"><tt class="xref docutils literal"><span class="pre">eval()</span></tt></a> and <a title="execfile" class="reference external" href="../library/functions.html#execfile"><tt class="xref docutils literal"><span class="pre">execfile()</span></tt></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>4.2. Exceptions<a class="headerlink" href="#exceptions" title="Permalink to this headline">¶</a></h2>
<span class="target" id="index-928"></span><p id="index-929">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 external" href="simple_stmts.html#raise"><tt class="xref docutils literal"><span class="pre">raise</span></tt></a> statement. Exception handlers are specified
with the <a class="reference external" href="compound_stmts.html#try"><tt class="xref docutils literal"><span class="pre">try</span></tt></a> ... <a class="reference external" href="compound_stmts.html#except"><tt class="xref docutils literal"><span class="pre">except</span></tt></a> statement. The <a class="reference external" href="compound_stmts.html#finally"><tt class="xref docutils literal"><span class="pre">finally</span></tt></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-930">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-931">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 backtrace, except when the exception is <a title="exceptions.SystemExit" class="reference external" href="../library/exceptions.html#exceptions.SystemExit"><tt class="xref docutils literal"><span class="pre">SystemExit</span></tt></a>.</p>
<p>Exceptions are identified by class instances. The <a class="reference external" href="compound_stmts.html#except"><tt class="xref docutils literal"><span class="pre">except</span></tt></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>
<p>Exceptions can also be identified by strings, in which case the
<a class="reference external" href="compound_stmts.html#except"><tt class="xref docutils literal"><span class="pre">except</span></tt></a> clause is selected by object identity. An arbitrary value can
be raised along with the identifying string which can be passed to the handler.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">Messages to exceptions 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 external" href="compound_stmts.html#try"><tt class="xref docutils literal"><span class="pre">try</span></tt></a> statement in section <a class="reference external" href="compound_stmts.html#try"><em>The try statement</em></a>
and <a class="reference external" href="simple_stmts.html#raise"><tt class="xref docutils literal"><span class="pre">raise</span></tt></a> statement in section <a class="reference external" href="simple_stmts.html#raise"><em>The raise statement</em></a>.</p>
<p class="rubric">Footnotes</p>
<table class="docutils footnote" frame="void" id="id3" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id1">[1]</a></td><td>This limitation occurs because the code that is executed by these operations is
not available at the time the module is compiled.</td></tr>
</tbody>
</table>
</div>
</div>
</div>
</div>
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<h3><a href="../contents.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference external" href="#">4. Execution model</a><ul>
<li><a class="reference external" href="#naming-and-binding">4.1. Naming and binding</a><ul>
<li><a class="reference external" href="#interaction-with-dynamic-features">4.1.1. Interaction with dynamic features</a></li>
</ul>
</li>
<li><a class="reference external" href="#exceptions">4.2. Exceptions</a></li>
</ul>
</li>
</ul>
<h4>Previous topic</h4>
<p class="topless"><a href="datamodel.html"
title="previous chapter">3. Data model</a></p>
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title="next chapter">5. Expressions</a></p>
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