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python-3.7.4-docs-html/_sources/reference/executionmodel.rst.txt
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python-3.7.4-docs-html/_sources/reference/executionmodel.rst.txt
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.. _execmodel:
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***************
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Execution model
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***************
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.. index::
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single: execution model
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pair: code; block
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.. _prog_structure:
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Structure of a program
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======================
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.. index:: block
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A Python program is constructed from code blocks.
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A :dfn:`block` 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 :option:`-c` option) is a code block. The string
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argument passed to the built-in functions :func:`eval` and :func:`exec` is a
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code block.
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.. index:: pair: execution; frame
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A code block is executed in an :dfn:`execution frame`. 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.
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.. _naming:
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Naming and binding
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==================
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.. index::
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single: namespace
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single: scope
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.. _bind_names:
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Binding of names
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----------------
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.. index::
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single: name
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pair: binding; name
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:dfn:`Names` refer to objects. Names are introduced by name binding operations.
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.. index:: single: from; import statement
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The following constructs bind names: formal parameters to functions,
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:keyword:`import` 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, :keyword:`for` loop header, or after
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:keyword:`!as` in a :keyword:`with` statement or :keyword:`except` clause.
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The :keyword:`!import` statement
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of the form ``from ... import *`` 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.
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A target occurring in a :keyword:`del` statement is also considered bound for
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this purpose (though the actual semantics are to unbind the name).
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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).
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.. index:: pair: free; variable
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If a name is bound in a block, it is a local variable of that block, unless
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declared as :keyword:`nonlocal` or :keyword:`global`. 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 :dfn:`free variable`.
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Each occurrence of a name in the program text refers to the :dfn:`binding` of
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that name established by the following name resolution rules.
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.. _resolve_names:
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Resolution of names
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-------------------
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.. index:: scope
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A :dfn:`scope` 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.
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.. index:: single: environment
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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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:dfn:`environment`.
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.. index::
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single: NameError (built-in exception)
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single: UnboundLocalError
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When a name is not found at all, a :exc:`NameError` 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 :exc:`UnboundLocalError` exception is raised.
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:exc:`UnboundLocalError` is a subclass of :exc:`NameError`.
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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.
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If the :keyword:`global` 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 :mod:`builtins`. 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 :keyword:`!global` statement must precede
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all uses of the name.
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The :keyword:`global` 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.
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.. XXX say more about "nonlocal" semantics here
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The :keyword:`nonlocal` statement causes corresponding names to refer
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to previously bound variables in the nearest enclosing function scope.
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:exc:`SyntaxError` is raised at compile time if the given name does not
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exist in any enclosing function scope.
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.. index:: module: __main__
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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 :mod:`__main__`.
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Class definition blocks and arguments to :func:`exec` and :func:`eval` 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::
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class A:
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a = 42
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b = list(a + i for i in range(10))
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.. _restrict_exec:
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Builtins and restricted execution
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---------------------------------
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.. index:: pair: restricted; execution
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.. impl-detail::
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Users should not touch ``__builtins__``; it is strictly an implementation
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detail. Users wanting to override values in the builtins namespace should
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:keyword:`import` the :mod:`builtins` module and modify its
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attributes appropriately.
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The builtins namespace associated with the execution of a code block
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is actually found by looking up the name ``__builtins__`` in its
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global namespace; this should be a dictionary or a module (in the
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latter case the module's dictionary is used). By default, when in the
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:mod:`__main__` module, ``__builtins__`` is the built-in module
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:mod:`builtins`; when in any other module, ``__builtins__`` is an
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alias for the dictionary of the :mod:`builtins` module itself.
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.. _dynamic-features:
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Interaction with dynamic features
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---------------------------------
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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::
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i = 10
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def f():
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print(i)
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i = 42
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f()
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.. XXX from * also invalid with relative imports (at least currently)
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The :func:`eval` and :func:`exec` 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. [#]_ The :func:`exec` and
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:func:`eval` 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.
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.. _exceptions:
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Exceptions
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==========
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.. index:: single: exception
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.. index::
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single: raise an exception
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single: handle an exception
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single: exception handler
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single: errors
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single: error handling
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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 *raised* at the point where the error is detected; it may be *handled* 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.
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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 :keyword:`raise` statement. Exception handlers are specified
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with the :keyword:`try` ... :keyword:`except` statement. The :keyword:`finally`
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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.
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.. index:: single: termination model
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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).
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.. index:: single: SystemExit (built-in exception)
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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 traceback, except when the exception is :exc:`SystemExit`.
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Exceptions are identified by class instances. The :keyword:`except` 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.
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.. note::
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Exception messages are not part of the Python API. Their contents may change
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from one version of Python to the next without warning and should not be
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relied on by code which will run under multiple versions of the interpreter.
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See also the description of the :keyword:`try` statement in section :ref:`try`
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and :keyword:`raise` statement in section :ref:`raise`.
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.. rubric:: Footnotes
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.. [#] This limitation occurs because the code that is executed by these operations
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is not available at the time the module is compiled.
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