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diff --git a/Doc/reference/executionmodel.rst b/Doc/reference/executionmodel.rst index 55ac01b..91ac72f 100644 --- a/Doc/reference/executionmodel.rst +++ b/Doc/reference/executionmodel.rst @@ -5,34 +5,8 @@ Execution model *************** -.. index:: - single: execution model - pair: code; block - -.. _prog_structure: - -Structure of a program -====================== - -.. index:: block - -A Python program is constructed from code blocks. -A :dfn:`block` is a piece of Python program text that is executed as a unit. -The following are blocks: a module, a function body, and a class definition. -Each command typed interactively is a block. A script file (a file given as -standard input to the interpreter or specified as a command line argument to the -interpreter) is a code block. A script command (a command specified on the -interpreter command line with the :option:`-c` option) is a code block. -A module run as a top level script (as module ``__main__``) from the command -line using a :option:`-m` argument is also a code block. The string -argument passed to the built-in functions :func:`eval` and :func:`exec` is a -code block. +.. index:: single: execution model -.. index:: pair: execution; frame - -A code block is executed in an :dfn:`execution frame`. A frame contains some -administrative information (used for debugging) and determines where and how -execution continues after the code block's execution has completed. .. _naming: @@ -40,61 +14,51 @@ Naming and binding ================== .. index:: + pair: code; block single: namespace single: scope -.. _bind_names: - -Binding of names ----------------- - .. index:: single: name pair: binding; name :dfn:`Names` refer to objects. Names are introduced by name binding operations. +Each occurrence of a name in the program text refers to the :dfn:`binding` of +that name established in the innermost function block containing the use. -.. index:: single: from; import statement - -The following constructs bind names: formal parameters to functions, -:keyword:`import` 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, :keyword:`for` loop header, or after -:keyword:`!as` in a :keyword:`with` statement or :keyword:`except` clause. -The :keyword:`!import` statement -of the form ``from ... import *`` binds all names defined in the imported -module, except those beginning with an underscore. This form may only be used -at the module level. - -A target occurring in a :keyword:`del` statement is also considered bound for -this purpose (though the actual semantics are to unbind the name). - -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). - -.. index:: pair: free; variable - -If a name is bound in a block, it is a local variable of that block, unless -declared as :keyword:`nonlocal` or :keyword:`global`. 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 :dfn:`free variable`. +.. index:: single: block -Each occurrence of a name in the program text refers to the :dfn:`binding` of -that name established by the following name resolution rules. +A :dfn:`block` 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 '**-c**' option) is a code block. The +file read by the built-in function :func:`execfile` is a code block. The string +argument passed to the built-in function :func:`eval` and to the :keyword:`exec` +statement is a code block. The expression read and evaluated by the built-in +function :func:`input` is a code block. -.. _resolve_names: +.. index:: pair: execution; frame -Resolution of names -------------------- +A code block is executed in an :dfn:`execution frame`. A frame contains some +administrative information (used for debugging) and determines where and how +execution continues after the code block's execution has completed. -.. index:: scope +.. index:: single: scope A :dfn:`scope` 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. +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:: + + class A: + a = 42 + b = list(a + i for i in range(10)) .. index:: single: environment @@ -102,84 +66,88 @@ 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 :dfn:`environment`. +.. index:: pair: free; variable + +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 :dfn:`free variable`. + .. index:: single: NameError (built-in exception) single: UnboundLocalError -When a name is not found at all, a :exc:`NameError` exception is raised. -If the current scope is a function scope, and the name refers to a local -variable that has not yet been bound to a value at the point where the name is -used, an :exc:`UnboundLocalError` exception is raised. -:exc:`UnboundLocalError` is a subclass of :exc:`NameError`. +When a name is not found at all, a :exc:`NameError` exception is raised. If the +name refers to a local variable that has not been bound, a +:exc:`UnboundLocalError` exception is raised. :exc:`UnboundLocalError` is a +subclass of :exc:`NameError`. + +.. index:: statement: from + +The following constructs bind names: formal parameters to functions, +:keyword:`import` 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, :keyword:`for` loop header, in the second +position of an :keyword:`except` clause header or after :keyword:`as` in a +:keyword:`with` statement. The :keyword:`import` statement +of the form ``from ... import *`` binds all names defined in the imported +module, except those beginning with an underscore. This form may only be used +at the module level. + +A target occurring in a :keyword:`del` 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 :exc:`SyntaxError`. + +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). 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 +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. -If the :keyword:`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 :mod:`builtins`. The -global namespace is searched first. If the name is not found there, the -builtins namespace is searched. The :keyword:`!global` statement must precede -all uses of the name. +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 :mod:`__builtin__`. 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. -The :keyword:`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. +.. index:: pair: restricted; execution -.. XXX say more about "nonlocal" semantics here +The builtins namespace associated with the execution of a code block is actually +found by looking up the name ``__builtins__`` 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 :mod:`__main__` module, ``__builtins__`` is +the built-in module :mod:`__builtin__` (note: no 's'); when in any other module, +``__builtins__`` is an alias for the dictionary of the :mod:`__builtin__` module +itself. ``__builtins__`` can be set to a user-created dictionary to create a +weak form of restricted execution. -The :keyword:`nonlocal` statement causes corresponding names to refer -to previously bound variables in the nearest enclosing function scope. -:exc:`SyntaxError` is raised at compile time if the given name does not -exist in any enclosing function scope. +.. impl-detail:: + + Users should not touch ``__builtins__``; it is strictly an implementation + detail. Users wanting to override values in the builtins namespace should + :keyword:`import` the :mod:`__builtin__` (no 's') module and modify its + attributes appropriately. .. index:: module: __main__ The namespace for a module is automatically created the first time a module is imported. The main module for a script is always called :mod:`__main__`. -Class definition blocks and arguments to :func:`exec` and :func:`eval` 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:: - - class A: - a = 42 - b = list(a + i for i in range(10)) - -.. _restrict_exec: - -Builtins and restricted execution ---------------------------------- - -.. index:: pair: restricted; execution - -.. impl-detail:: - - Users should not touch ``__builtins__``; it is strictly an implementation - detail. Users wanting to override values in the builtins namespace should - :keyword:`import` the :mod:`builtins` module and modify its - attributes appropriately. +The :keyword:`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. -The builtins namespace associated with the execution of a code block -is actually found by looking up the name ``__builtins__`` 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 -:mod:`__main__` module, ``__builtins__`` is the built-in module -:mod:`builtins`; when in any other module, ``__builtins__`` is an -alias for the dictionary of the :mod:`builtins` module itself. +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. .. _dynamic-features: @@ -187,23 +155,30 @@ alias for the dictionary of the :mod:`builtins` module itself. Interaction with dynamic features --------------------------------- -Name resolution of free variables occurs at runtime, not at compile time. -This means that the following code will print 42:: +There are several cases where Python statements are illegal when used in +conjunction with nested scopes that contain free variables. + +If a variable is referenced in an enclosing scope, it is illegal to delete the +name. An error will be reported at compile time. - i = 10 - def f(): - print(i) - i = 42 - f() +If the wild card form of import --- ``import *`` --- is used in a function and +the function contains or is a nested block with free variables, the compiler +will raise a :exc:`SyntaxError`. -.. XXX from * also invalid with relative imports (at least currently) +If :keyword:`exec` is used in a function and the function contains or is a +nested block with free variables, the compiler will raise a :exc:`SyntaxError` +unless the exec explicitly specifies the local namespace for the +:keyword:`exec`. (In other words, ``exec obj`` would be illegal, but ``exec obj +in ns`` would be legal.) -The :func:`eval` and :func:`exec` 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. [#]_ The :func:`exec` and -:func:`eval` functions have optional arguments to override the global and local -namespace. If only one namespace is specified, it is used for both. +The :func:`eval`, :func:`execfile`, and :func:`input` functions and the +:keyword:`exec` 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. [#]_ The :keyword:`exec` statement and the +:func:`eval` and :func:`execfile` functions have optional arguments to override +the global and local namespace. If only one namespace is specified, it is used +for both. .. _exceptions: @@ -245,24 +220,28 @@ re-entering the offending piece of code from the top). 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 :exc:`SystemExit`. +a stack backtrace, except when the exception is :exc:`SystemExit`. Exceptions are identified by class instances. The :keyword:`except` 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. +Exceptions can also be identified by strings, in which case the +:keyword:`except` clause is selected by object identity. An arbitrary value can +be raised along with the identifying string which can be passed to the handler. + .. note:: - 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 + 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. See also the description of the :keyword:`try` statement in section :ref:`try` and :keyword:`raise` statement in section :ref:`raise`. - .. rubric:: Footnotes -.. [#] This limitation occurs because the code that is executed by these operations - is not available at the time the module is compiled. +.. [#] This limitation occurs because the code that is executed by these operations is + not available at the time the module is compiled. + |