python - "local variable referenced before assignment"——只有功能？

Question

采用以下代码:

import something

def Foo():
    something = something.SomeClass()
    return something

...这显然不是有效代码:

UnboundLocalError: local variable 'something' referenced before assignment

...因为局部变量 something 被创建，但没有赋值，在 = 的 RHS 被评估之前。 (例如，请参见 this related answer's comment。)这对我来说似乎有点奇怪，但可以肯定的是，我会接受它。现在，为什么下面的代码有效？

class Foo(object):
    something = something.SomeClass()

我的理解是 class 定义的内部本质上是一个范围:

The class’s suite is then executed in a new execution frame (see section Naming and binding), using a newly created local namespace and the original global namespace.

那么，为什么该代码的行为与函数的行为不同？

Answer 1

来自python class documentation :

Class definitions place yet another namespace in the local scope.

A special quirk of Python is that – if no global statement is in effect – assignments to names 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.)

所以在函数(或作用域)内，赋值创建一个局部未绑定(bind)变量，在绑定(bind)之前访问该变量，而在类定义中，它在赋值时在该类的“命名空间”字典中创建一个条目，允许将 something 解析为外部命名空间(模块命名空间)。