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 execfile()
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 NameError
exception is raised. If the name refers to a local variable that has not been bound,
a UnboundLocalError
exception is raised. UnboundLocalError is a subclass of NameError.
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, for loop header, or in the second
position of an except clause header. The 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 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 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). Normally, the
__builtins__ namespace is the dictionary of the built-in module __builtin__ (note: no `s'). If it isn't, restricted execution
mode is in effect.
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.
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