|Python Language Reference Manual|
by Guido van Rossum and Fred L. Drake, Jr.
Paperback (6"x9"), 120 pages
RRP £12.95 ($19.95)
Sales of this book support the Python Software Foundation! Get a printed copy>>>
4.1 Naming and binding
Names refer to objects. Names are introduced by name binding operations. Each occurrence of a name in the program text refers to the binding of that name established in the innermost function block containing the use.
A 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
is a code block. The string argument passed to the built-in function
eval() and to the
exec statement is a code block.
The expression read and evaluated by the built-in function
input() is a code block.
A code block is executed in an 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.
A 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.
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 environment.
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 free variable.
When a name is not found at all, a
exception is raised. If the name
refers to a local variable that has not been bound, an
UnboundLocalError is a subclass of
The following constructs bind names: formal parameters to functions,
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,
header, or in the second position of an
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
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
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 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 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 builtin namespace, the namespace of the module ‘__builtin__’. The global namespace is searched first. If the name is not found there, the builtin namespace is searched. The global statement must precede all uses of the name.
The built-in 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
__builtins__ is the built-in module
‘__builtin__’ (note: no `s'); when in any other module,
__builtins__ is an alias for the dictionary of the
‘__builtin__’ module itself.
__builtins__ can be set
to a user-created dictionary to create a weak form of restricted
Users should not touch
__builtins__; it is strictly an
implementation detail. Users wanting to override values in the
built-in namespace should
import the ‘__builtin__’
(no `s') module and modify its attributes appropriately.
The namespace for a module is automatically created the first time a module is imported. The main module for a script is always called ‘__main__’ .
The 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.
|ISBN 0954161785||Python Language Reference Manual||See the print edition|