范围规则的简短描述? [英] Short description of the scoping rules?

问题描述

什么是Python范围界定规则?

如果我有一些代码:

code1
class Foo:
   code2
   def spam.....
      code3
      for code4..:
       code5
       x()

在哪里找到x?一些可能的选择包括以下列表:

1. 在随附的源文件中
2. 在类命名空间中
3. 在函数定义中
4. 在for循环索引变量中
5. 在for循环内

在执行过程中，当功能spam被传递到其他地方时，也存在上下文.也许 lambda函数传递的方式有所不同吗?

某处必须有一个简单的参考或算法.对于中级Python程序员而言，这是一个令人困惑的世界.

解决方案

实际上，这是 Antti的答案或此处).

What exactly are the Python scoping rules?

If I have some code:

code1
class Foo:
   code2
   def spam.....
      code3
      for code4..:
       code5
       x()

Where is x found? Some possible choices include the list below:

1. In the enclosing source file
2. In the class namespace
3. In the function definition
4. In the for loop index variable
5. Inside the for loop

Also there is the context during execution, when the function spam is passed somewhere else. And maybe lambda functions pass a bit differently?

There must be a simple reference or algorithm somewhere. It's a confusing world for intermediate Python programmers.

解决方案

Actually, a concise rule for Python Scope resolution, from Learning Python, 3rd. Ed.. (These rules are specific to variable names, not attributes. If you reference it without a period, these rules apply.)

LEGB Rule

• Local — Names assigned in any way within a function (def or lambda), and not declared global in that function

• Enclosing-function — Names assigned in the local scope of any and all statically enclosing functions (def or lambda), from inner to outer

• Global (module) — Names assigned at the top-level of a module file, or by executing a global statement in a def within the file

• Built-in (Python) — Names preassigned in the built-in names module: open, range, SyntaxError, etc

So, in the case of

code1
class Foo:
    code2
    def spam():
        code3
        for code4:
            code5
            x()

The for loop does not have its own namespace. In LEGB order, the scopes would be

• L: Local in def spam (in code3, code4, and code5)
• E: Any enclosing functions (if the whole example were in another def)
• G: Were there any x declared globally in the module (in code1)?
• B: Any builtin x in Python.

x will never be found in code2 (even in cases where you might expect it would, see Antti's answer or here).

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