4.1 !!! 代码块, 运行结构框架和命名空间 Naming and binding

Namesrefer 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 blockis 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.

一个代码块是一个可以作为一个单元执行的Python程序文本, 像模块, 类定义或函数体. 有些代码块(如模块)通常只执行一次, 其它(例如函数体)可能会执行多次.代码块可以直接包含其它代码块, 也可以调用其它代码块(可能包括也可能不包括它们), 例如调用函数.

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.

每个代码块在一个运行结构框架中执行.运行结构框架包括一个管理信息(用于调试),决定执行完代码块后在哪继续执行和怎么执行, 并且(可能也是最重要的)定义两个名字空间, 对于所执行代码块有效的局部和全局名字空间.

A scopedefines 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 UnboundLocalErrorexception 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.