.. index:: single: execution model pair: code; block
.. 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. The string argument passed to the built-in functions :func:`eval` and :func:`exec` is a code block.
.. 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.
.. index:: single: namespace single: scope
.. index:: single: name pair: binding; name
:dfn:`Names` refer to objects. Names are introduced by name binding operations.
.. 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`.
Each occurrence of a name in the program text refers to the :dfn:`binding` of that name established by the following name resolution rules.
.. index:: 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.
.. index:: single: environment
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:: 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`.
If a name binding operation occurs anywhere within a code block, all uses of the name within the block are treated as references to the current block. This can lead to errors when a name is used within a block before it is bound. This rule is subtle. Python lacks declarations and allows name binding operations to occur anywhere within a code block. The local variables of a code block can be determined by scanning the entire text of the block for name binding operations.
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.
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 :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.
.. 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))
.. 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 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.
Name resolution of free variables occurs at runtime, not at compile time. This means that the following code will print 42:
i = 10 def f(): print(i) i = 42 f()
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. [1] 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.
.. index:: single: exception
.. index:: single: raise an exception single: handle an exception single: exception handler single: errors single: error handling
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 raised at the point where the error is detected; it may be handled by the surrounding code block or by any code block that directly or indirectly invoked the code block where the error occurred.
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 :keyword:`raise` statement. Exception handlers are specified with the :keyword:`try` ... :keyword:`except` statement. The :keyword:`finally` 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.
.. index:: single: termination model
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).
.. index:: single: SystemExit (built-in exception)
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`.
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.
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 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`.
Footnotes
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