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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 ‘<strong>-c</strong>‘ 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"><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"><span class="pre">exec()</span></code></a> is a
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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 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"><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"><span class="pre">for</span></code></a> loop header, or after
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<a class="reference internal" href="compound_stmts.html#as"><code class="xref std std-keyword docutils literal"><span class="pre">as</span></code></a> in a <a class="reference internal" href="compound_stmts.html#with"><code class="xref std std-keyword docutils literal"><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"><span class="pre">except</span></code></a> clause.
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The <a class="reference internal" href="simple_stmts.html#import"><code class="xref std std-keyword docutils literal"><span class="pre">import</span></code></a> statement
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of the form <code class="docutils literal"><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"><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"><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"><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 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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<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"><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"><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"><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"><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"><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
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namespace. Names are resolved in the top-level namespace by searching the
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global namespace, i.e. the namespace of the module containing the code block,
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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"><span class="pre">builtins</span></code></a>. The
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global namespace is searched first. If the name is not found there, the
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builtins namespace is searched. The <a class="reference internal" href="simple_stmts.html#global"><code class="xref std std-keyword docutils literal"><span class="pre">global</span></code></a> statement must precede
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all uses of the name.</p>
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<p>The <a class="reference internal" href="simple_stmts.html#global"><code class="xref std std-keyword docutils literal"><span class="pre">global</span></code></a> statement has the same scope as a name binding operation
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in the same block. If the nearest enclosing scope for a free variable contains
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a global statement, the free variable is treated as a global.</p>
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<p>The <a class="reference internal" href="simple_stmts.html#nonlocal"><code class="xref std std-keyword docutils literal"><span class="pre">nonlocal</span></code></a> statement causes corresponding names to refer
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to previously bound variables in the nearest enclosing function scope.
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<a class="reference internal" href="../library/exceptions.html#SyntaxError" title="SyntaxError"><code class="xref py py-exc docutils literal"><span class="pre">SyntaxError</span></code></a> is raised at compile time if the given name does not
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exist in any enclosing function scope.</p>
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<p id="index-10">The namespace for a module is automatically created the first time a module is
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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"><span class="pre">__main__</span></code></a>.</p>
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<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"><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"><span class="pre">eval()</span></code></a> are
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special in the context of name resolution.
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A class definition is an executable statement that may use and define names.
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These references follow the normal rules for name resolution with an exception
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that unbound local variables are looked up in the global namespace.
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The namespace of the class definition becomes the attribute dictionary of
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the class. The scope of names defined in a class block is limited to the
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class block; it does not extend to the code blocks of methods – this includes
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comprehensions and generator expressions since they are implemented using a
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function scope. This means that the following will fail:</p>
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<div class="highlight-python3"><div class="highlight"><pre><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>
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<div class="section" id="builtins-and-restricted-execution">
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<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>
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<p id="index-11">The builtins namespace associated with the execution of a code block is actually
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found by looking up the name <code class="docutils literal"><span class="pre">__builtins__</span></code> in its global namespace; this
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should be a dictionary or a module (in the latter case the module’s dictionary
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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"><span class="pre">__main__</span></code></a> module, <code class="docutils literal"><span class="pre">__builtins__</span></code> is
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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"><span class="pre">builtins</span></code></a>; when in any other module,
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<code class="docutils literal"><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"><span class="pre">builtins</span></code></a> module
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itself. <code class="docutils literal"><span class="pre">__builtins__</span></code> can be set to a user-created dictionary to create a
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weak form of restricted execution.</p>
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<div class="impl-detail compound">
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<p><strong>CPython implementation detail:</strong> Users should not touch <code class="docutils literal"><span class="pre">__builtins__</span></code>; it is strictly an implementation
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detail. Users wanting to override values in the builtins namespace should
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<a class="reference internal" href="simple_stmts.html#import"><code class="xref std std-keyword docutils literal"><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"><span class="pre">builtins</span></code></a> module and modify its
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attributes appropriately.</p>
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<div class="section" id="interaction-with-dynamic-features">
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<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>
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<p>Name resolution of free variables occurs at runtime, not at compile time.
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This means that the following code will print 42:</p>
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<div class="highlight-python3"><div class="highlight"><pre><span class="n">i</span> <span class="o">=</span> <span class="mi">10</span>
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<span class="k">def</span> <span class="nf">f</span><span class="p">():</span>
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<span class="nb">print</span><span class="p">(</span><span class="n">i</span><span class="p">)</span>
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<span class="n">i</span> <span class="o">=</span> <span class="mi">42</span>
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<span class="n">f</span><span class="p">()</span>
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<p>There are several cases where Python statements are illegal when used in
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conjunction with nested scopes that contain free variables.</p>
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<p>If a variable is referenced in an enclosing scope, it is illegal to delete the
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name. An error will be reported at compile time.</p>
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<p>The <a class="reference internal" href="../library/functions.html#eval" title="eval"><code class="xref py py-func docutils literal"><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"><span class="pre">exec()</span></code></a> functions do not have access to the full
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environment for resolving names. Names may be resolved in the local and global
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namespaces of the caller. Free variables are not resolved in the nearest
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enclosing namespace, but in the global namespace. <a class="footnote-reference" 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"><span class="pre">exec()</span></code></a> and
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<a class="reference internal" href="../library/functions.html#eval" title="eval"><code class="xref py py-func docutils literal"><span class="pre">eval()</span></code></a> functions have optional arguments to override the global and local
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namespace. If only one namespace is specified, it is used for both.</p>
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<div class="section" id="exceptions">
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<span id="id2"></span><h2>4.3. Exceptions<a class="headerlink" href="#exceptions" title="Permalink to this headline">¶</a></h2>
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<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
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block in order to handle errors or other exceptional conditions. An exception
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is <em>raised</em> at the point where the error is detected; it may be <em>handled</em> by the
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surrounding code block or by any code block that directly or indirectly invoked
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the code block where the error occurred.</p>
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<p>The Python interpreter raises an exception when it detects a run-time error
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(such as division by zero). A Python program can also explicitly raise an
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exception with the <a class="reference internal" href="simple_stmts.html#raise"><code class="xref std std-keyword docutils literal"><span class="pre">raise</span></code></a> statement. Exception handlers are specified
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with the <a class="reference internal" href="compound_stmts.html#try"><code class="xref std std-keyword docutils literal"><span class="pre">try</span></code></a> ... <a class="reference internal" href="compound_stmts.html#except"><code class="xref std std-keyword docutils literal"><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"><span class="pre">finally</span></code></a>
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clause of such a statement can be used to specify cleanup code which does not
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handle the exception, but is executed whether an exception occurred or not in
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the preceding code.</p>
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<p id="index-14">Python uses the “termination” model of error handling: an exception handler can
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find out what happened and continue execution at an outer level, but it cannot
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repair the cause of the error and retry the failing operation (except by
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re-entering the offending piece of code from the top).</p>
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<p id="index-15">When an exception is not handled at all, the interpreter terminates execution of
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the program, or returns to its interactive main loop. In either case, it prints
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a stack backtrace, except when the exception is <a class="reference internal" href="../library/exceptions.html#SystemExit" title="SystemExit"><code class="xref py py-exc docutils literal"><span class="pre">SystemExit</span></code></a>.</p>
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<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"><span class="pre">except</span></code></a> clause is
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selected depending on the class of the instance: it must reference the class of
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the instance or a base class thereof. The instance can be received by the
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handler and can carry additional information about the exceptional condition.</p>
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<div class="admonition note">
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<p class="first admonition-title">Note</p>
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<p class="last">Exception messages are not part of the Python API. Their contents may change
241
from one version of Python to the next without warning and should not be
242
relied on by code which will run under multiple versions of the interpreter.</p>
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<p>See also the description of the <a class="reference internal" href="compound_stmts.html#try"><code class="xref std std-keyword docutils literal"><span class="pre">try</span></code></a> statement in section <a class="reference internal" href="compound_stmts.html#try"><span>The try statement</span></a>
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and <a class="reference internal" href="simple_stmts.html#raise"><code class="xref std std-keyword docutils literal"><span class="pre">raise</span></code></a> statement in section <a class="reference internal" href="simple_stmts.html#raise"><span>The raise statement</span></a>.</p>
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<p class="rubric">Footnotes</p>
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<table class="docutils footnote" frame="void" id="id3" rules="none">
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<colgroup><col class="label" /><col /></colgroup>
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<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
251
is not available at the time the module is compiled.</td></tr>
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<div class="sphinxsidebar" role="navigation" aria-label="main navigation">
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<div class="sphinxsidebarwrapper">
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<h3><a href="../contents.html">Table Of Contents</a></h3>
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<li><a class="reference internal" href="#">4. Execution model</a><ul>
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<li><a class="reference internal" href="#structure-of-a-program">4.1. Structure of a program</a></li>
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<li><a class="reference internal" href="#naming-and-binding">4.2. Naming and binding</a><ul>
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<li><a class="reference internal" href="#binding-of-names">4.2.1. Binding of names</a></li>
269
<li><a class="reference internal" href="#resolution-of-names">4.2.2. Resolution of names</a></li>
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<li><a class="reference internal" href="#builtins-and-restricted-execution">4.2.3. Builtins and restricted execution</a></li>
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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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<li><a class="reference internal" href="#exceptions">4.3. Exceptions</a></li>
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Last updated on Jan 22, 2016.
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