mirror of https://github.com/python/cpython
Merge issue #24129 from 3.4
This commit is contained in:
Nick Coghlan
commit
73c6f64a5e
|
|
@ -5,29 +5,18 @@
|
|||
Execution model
|
||||
***************
|
||||
|
||||
.. index:: single: execution model
|
||||
|
||||
|
||||
.. _naming:
|
||||
|
||||
Naming and binding
|
||||
==================
|
||||
|
||||
.. index::
|
||||
single: execution model
|
||||
pair: code; block
|
||||
single: namespace
|
||||
single: scope
|
||||
|
||||
.. index::
|
||||
single: name
|
||||
pair: binding; name
|
||||
.. _prog_structure:
|
||||
|
||||
: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.
|
||||
Structure of a programm
|
||||
=======================
|
||||
|
||||
.. 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
|
||||
|
|
@ -43,43 +32,25 @@ 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
|
||||
.. _naming:
|
||||
|
||||
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. 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:: 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:: pair: free; variable
|
||||
|
||||
If a name is bound in a block, it is a local variable of that block, unless
|
||||
declared as :keyword:`nonlocal`. 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`.
|
||||
Naming and binding
|
||||
==================
|
||||
|
||||
.. index::
|
||||
single: NameError (built-in exception)
|
||||
single: UnboundLocalError
|
||||
single: namespace
|
||||
single: scope
|
||||
|
||||
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, an
|
||||
:exc:`UnboundLocalError` exception is raised. :exc:`UnboundLocalError` is a
|
||||
subclass of :exc:`NameError`.
|
||||
.. _bind_names:
|
||||
|
||||
Binding of names
|
||||
----------------
|
||||
|
||||
.. index::
|
||||
single: name
|
||||
pair: binding; name
|
||||
|
||||
:dfn:`Names` refer to objects. Names are introduced by name binding operations.
|
||||
|
||||
.. index:: statement: from
|
||||
|
||||
|
|
@ -99,6 +70,46 @@ 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.
|
||||
|
||||
.. _resolve_names:
|
||||
|
||||
Resolution of names
|
||||
-------------------
|
||||
|
||||
.. 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
|
||||
|
|
@ -115,7 +126,41 @@ 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.
|
||||
|
||||
.. XXX document "nonlocal" semantics here
|
||||
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.
|
||||
|
||||
.. XXX say more about "nonlocal" semantics here
|
||||
|
||||
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))
|
||||
|
||||
.. _restrict_exec:
|
||||
|
||||
Builtins and restricted execution
|
||||
---------------------------------
|
||||
|
||||
.. index:: pair: restricted; execution
|
||||
|
||||
|
|
@ -135,26 +180,20 @@ weak form of restricted execution.
|
|||
:keyword:`import` the :mod:`builtins` 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__`.
|
||||
|
||||
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.
|
||||
|
||||
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:
|
||||
|
||||
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::
|
||||
|
||||
i = 10
|
||||
def f():
|
||||
print(i)
|
||||
i = 42
|
||||
f()
|
||||
|
||||
There are several cases where Python statements are illegal when used in
|
||||
conjunction with nested scopes that contain free variables.
|
||||
|
||||
|
|
|
|||
|
|
@ -836,6 +836,7 @@ Christopher Tur Lesniewski-Laas
|
|||
Alain Leufroy
|
||||
Mark Levinson
|
||||
Mark Levitt
|
||||
Ivan Levkivskyi
|
||||
William Lewis
|
||||
Akira Li
|
||||
Xuanji Li
|
||||
|
|
|
|||
|
|
@ -40,6 +40,11 @@ Library
|
|||
Documentation
|
||||
-------------
|
||||
|
||||
- Issue #24129: Clarify the reference documentation for name resolution.
|
||||
This includes removing the assumption that readers will be familiar with the
|
||||
name resolution scheme Python used prior to the introduction of lexical
|
||||
scoping for function namespaces. Patch by Ivan Levkivskyi.
|
||||
|
||||
- Issue #20769: Improve reload() docs. Patch by Dorian Pula.
|
||||
|
||||
- Issue #23589: Remove duplicate sentence from the FAQ. Patch by Yongzhi Pan.
|
||||
|
|
|
|||
Loading…
Reference in New Issue