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<div class="section" id="execution-model">
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<span id="execmodel"></span><h1>4. Execution model<a class="headerlink" href="#execution-model" title="Permalink to this headline">¶</a></h1>
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<div class="section" id="structure-of-a-program">
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<span id="prog-structure"></span><span id="index-0"></span><h2>4.1. Structure of a program<a class="headerlink" href="#structure-of-a-program" title="Permalink to this headline">¶</a></h2>
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<p id="index-1">A Python program is constructed from code blocks.
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A <em class="dfn">block</em> is a piece of Python program text that is executed as a unit.
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The following are blocks: a module, a function body, and a class definition.
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Each command typed interactively is a block. A script file (a file given as
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standard input to the interpreter or specified as a command line argument to the
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interpreter) is a code block. A script command (a command specified on the
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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
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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
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code block.</p>
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<p id="index-2">A code block is executed in an <em class="dfn">execution frame</em>. A frame contains some
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administrative information (used for debugging) and determines where and how
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execution continues after the code block’s execution has completed.</p>
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</div>
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<div class="section" id="naming-and-binding">
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<span id="naming"></span><h2>4.2. Naming and binding<a class="headerlink" href="#naming-and-binding" title="Permalink to this headline">¶</a></h2>
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<div class="section" id="binding-of-names">
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<span id="bind-names"></span><span id="index-3"></span><h3>4.2.1. Binding of names<a class="headerlink" href="#binding-of-names" title="Permalink to this headline">¶</a></h3>
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<p id="index-4"><em class="dfn">Names</em> refer to objects. Names are introduced by name binding operations.</p>
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<p id="index-5">The following constructs bind names: formal parameters to functions,
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<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
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class or function name in the defining block), and targets that are identifiers
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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
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<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.
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The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">import</span></code> statement
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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
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module, except those beginning with an underscore. This form may only be used
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at the module level.</p>
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<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
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this purpose (though the actual semantics are to unbind the name).</p>
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<p>Each assignment or import statement occurs within a block defined by a class or
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function definition or at the module level (the top-level code block).</p>
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<p id="index-6">If a name is bound in a block, it is a local variable of that block, unless
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||
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
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the module level, it is a global variable. (The variables of the module code
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block are local and global.) If a variable is used in a code block but not
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||
defined there, it is a <em class="dfn">free variable</em>.</p>
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||
<p>Each occurrence of a name in the program text refers to the <em class="dfn">binding</em> of
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||
that name established by the following name resolution rules.</p>
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</div>
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<div class="section" id="resolution-of-names">
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<span id="resolve-names"></span><h3>4.2.2. Resolution of names<a class="headerlink" href="#resolution-of-names" title="Permalink to this headline">¶</a></h3>
|
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<p id="index-7">A <em class="dfn">scope</em> defines the visibility of a name within a block. If a local
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||
variable is defined in a block, its scope includes that block. If the
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definition occurs in a function block, the scope extends to any blocks contained
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within the defining one, unless a contained block introduces a different binding
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for the name.</p>
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<p id="index-8">When a name is used in a code block, it is resolved using the nearest enclosing
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scope. The set of all such scopes visible to a code block is called the block’s
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<em class="dfn">environment</em>.</p>
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<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.
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If the current scope is a function scope, and the name refers to a local
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variable that has not yet been bound to a value at the point where the name is
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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.
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||
<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>
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||
<p>If a name binding operation occurs anywhere within a code block, all uses of the
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||
name within the block are treated as references to the current block. This can
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||
lead to errors when a name is used within a block before it is bound. This rule
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||
is subtle. Python lacks declarations and allows name binding operations to
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||
occur anywhere within a code block. The local variables of a code block can be
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||
determined by scanning the entire text of the block for name binding operations.</p>
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||
<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
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||
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>
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||
<span class="n">a</span> <span class="o">=</span> <span class="mi">42</span>
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<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>4.2.3. 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>4.2.4. 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>4.3. 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>
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|
||
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<li><a class="reference internal" href="#interaction-with-dynamic-features">4.2.4. Interaction with dynamic features</a></li>
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