Python 作用域和命名空间

9.2 Python 作用域和命名空间 Python Scopes and Name Spaces

Before introducing classes, I first have to tell you something about Python's scope rules. Class definitions play some neat tricks with namespaces, and you need to know how scopes and namespaces work to fully understand what's going on. Incidentally, knowledge about this subject is useful for any advanced Python programmer.

在介绍类之前，我首先介绍一些有关 Python 作用域的规则：类的定义非常巧妙的运用了命名空间，要完全理解接下来的知识，需要先理解作用域和命名空间的工作原理。另外，这一切的知识对于任何高级 Python 程序员都非常有用。

Let's begin with some definitions.

我们从一些定义开始。

A namespace is a mapping from names to objects. Most namespaces are currently implemented as Python dictionaries, but that's normally not noticeable in any way (except for performance), and it may change in the future. Examples of namespaces are: the set of built-in names (functions such as abs(), and built-in exception names); the global names in a module; and the local names in a function invocation. In a sense the set of attributes of an object also form a namespace. The important thing to know about namespaces is that there is absolutely no relation between names in different namespaces; for instance, two different modules may both define a function ``maximize'' without confusion -- users of the modules must prefix it with the module name.

命名空间是从命名到对象的映射。当前命名空间主要是通过 Python 字典实现的，不过通常不关心具体的实现方式（除非出于性能考虑），以后也有可能会改变其实现方式。以下有一些命名空间的例子：内置命名（像 abs() 这样的函数，以及内置异常名）集，模块中的全局命名，函数调用中的局部命名。某种意义上讲对象的属性集也是一个命名空间。关于命名空间需要了解的一件很重要的事就是不同命名空间中的命名没有任何联系，例如两个不同的模块可能都会定义一个名为“maximize”的函数而不会发生混淆－－用户必须以模块名为前缀来引用它们。

By the way, I use the word attribute for any name following a dot -- for example, in the expression z.real, real is an attribute of the object z. Strictly speaking, references to names in modules are attribute references: in the expression modname.funcname, modname is a module object and funcname is an attribute of it. In this case there happens to be a straightforward mapping between the module's attributes and the global names defined in the module: they share the same namespace! ^9.1

顺便提一句，我称 Python 中任何一个“.”之后的命名为属性－－例如，表达式 z.real 中的 real 是对象 z 的一个属性。严格来讲，从模块中引用命名是引用属性：表达式 modname.funcname 中， modname 是一个模块对象， funcname 是它的一个属性。因此，模块的属性和模块中的全局命名有直接的映射关系：它们共享同一命名空间！ ^9.2

Attributes may be read-only or writable. In the latter case, assignment to attributes is possible. Module attributes are writable: you can write "modname.the_answer = 42". Writable attributes may also be deleted with the del statement. For example, "del modname.the_answer" will remove the attribute the_answer from the object named by modname.

属性可以是只读过或写的。后一种情况下，可以对属性赋值。你可以这样作： "modname.the_answer = 42"。可写的属性也可以用 del 语句删除。例如："del modname.the_answer" 会从 modname 对象中删除 the_answer 属性。

Name spaces are created at different moments and have different lifetimes. The namespace containing the built-in names is created when the Python interpreter starts up, and is never deleted. The global namespace for a module is created when the module definition is read in; normally, module namespaces also last until the interpreter quits. The statements executed by the top-level invocation of the interpreter, either read from a script file or interactively, are considered part of a module called __main__, so they have their own global namespace. (The built-in names actually also live in a module; this is called __builtin__.)

不同的命名空间在不同的时刻创建，有不同的生存期。包含内置命名的命名空间在 Python 解释器启动时创建，会一直保留，不被删除。模块的全局命名空间在模块定义被读入时创建，通常，模块命名空间也会一直保存到解释器退出。由解释器在最高层调用执行的语句，不管它是从脚本文件中读入还是来自交互式输入，都是 __main__ 模块的一部分，所以它们也拥有自己的命名空间。（内置命名也同样被包含在一个模块中，它被称作 __builtin__ 。）

The local namespace for a function is created when the function is called, and deleted when the function returns or raises an exception that is not handled within the function. (Actually, forgetting would be a better way to describe what actually happens.) Of course, recursive invocations each have their own local namespace.

当函数被调用时创建一个局部命名空间，函数反正返回过抛出一个未在函数内处理的异常时删除。（实际上，说是遗忘更为贴切）。当然，每一个递归调用拥有自己的命名空间。

A scope is a textual region of a Python program where a namespace is directly accessible. ``Directly accessible'' here means that an unqualified reference to a name attempts to find the name in the namespace.

作用域是指 Python 程序可以直接访问到的命名空间。“直接访问”在这里意味着访问命名空间中的命名时无需加入附加的修饰符。

Although scopes are determined statically, they are used dynamically. At any time during execution, there are at least three nested scopes whose namespaces are directly accessible: the innermost scope, which is searched first, contains the local names; the namespaces of any enclosing functions, which are searched starting with the nearest enclosing scope; the middle scope, searched next, contains the current module's global names; and the outermost scope (searched last) is the namespace containing built-in names.

尽管作用域是静态定义，在使用时他们都是动态的。每次执行时，至少有三个命名空间可以直接访问的作用域嵌套在一起：包含局部命名的使用域在最里面，首先被搜索；其次搜索的是中层的作用域，这里包含了同级的函数；最后搜索最外面的作用域，它包含内置命名。

If a name is declared global, then all references and assignments go directly to the middle scope containing the module's global names. Otherwise, all variables found outside of the innermost scope are read-only.

如果一个命名声明为全局的，那么所有的赋值和引用都直接针对包含模全局命名的中级作用域。另外，从外部访问到的所有内层作用域的变量都是只读的。

Usually, the local scope references the local names of the (textually) current function. Outside of functions, the local scope references the same namespace as the global scope: the module's namespace. Class definitions place yet another namespace in the local scope.

从文本意义上讲，局部作用域引用当前函数的命名。在函数之外，局部作用域与全局使用域引用同一命名空间：模块命名空间。类定义也是局部作用域中的另一个命名空间。

It is important to realize that scopes are determined textually: the global scope of a function defined in a module is that module's namespace, no matter from where or by what alias the function is called. On the other hand, the actual search for names is done dynamically, at run time -- however, the language definition is evolving towards static name resolution, at ``compile'' time, so don't rely on dynamic name resolution! (In fact, local variables are already determined statically.)

作用域决定于源程序的文本：一个定义于某模块中的函数的全局作用域是该模块的命名空间，而不是该函数的别名被定义或调用的位置，了解这一点非常重要。另一方面，命名的实际搜索过程是动态的，在运行时确定的——然而，Python 语言也在不断发展，以后有可能会成为静态的“编译”时确定，所以不要依赖于动态解析！（事实上，局部变量已经是静态确定了。）

A special quirk of Python is that assignments always go into the innermost scope. Assignments do not copy data -- they just bind names to objects. The same is true for deletions: the statement "del x" removes the binding of x from the namespace referenced by the local scope. In fact, all operations that introduce new names use the local scope: in particular, import statements and function definitions bind the module or function name in the local scope. (The global statement can be used to indicate that particular variables live in the global scope.)

Python 的一个特别之处在于其赋值操作总是在最里层的作用域。赋值不会复制数据——只是将命名绑定到对象。删除也是如此："del x" 只是从局部作用域的命名空间中删除命名 x 。事实上，所有引入新命名的操作都作用于局部作用域。特别是 import 语句和函数定将模块名或函数绑定于局部作用域。（可以使用 global 语句将变量引入到全局作用域。）

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注脚

... namespace!^9.1

Except for one thing. Module objects have a secret read-only attribute called __dict__ which returns the dictionary used to implement the module's namespace; the name __dict__ is an attribute but not a global name. Obviously, using this violates the abstraction of namespace implementation, and should be restricted to things like post-mortem debuggers.

... 是它的一个属性。因此，模块的属性和模块中的全局命名有直接的映射关系：它们共享同一命名空间！^9.2

有一个例外。模块对象有一个隐秘的只读对象，名为 __dict__ ，它返回用于实现模块命名空间的字典，命名 __dict__ 是一个属性而非全局命名。显然，使用它违反了命名空间实现的抽象原则，应该被严格限制于调试中。