Python 中自由变量的搜索顺序是什么？

发布于 2024-09-12 09:01:25 字数 711 浏览 5 评论 0原文

具体来说，自由变量如何在类方法的定义中绑定？它可能是这样的：

封闭函数（临时）作用域=>生成闭包
全局（永久）范围=>不生成闭包（只需在方法体执行时查找它）
raise UnboundLocalError()

这里有两个例子：

globalname = 0
class Test(object):
    def method(self):
        print globalname
        print Test

def outer():
    localname = 1
    class Test(object):
        def method(self):
            print globalname
            print localname
            print Test
    return Test

Test().method.__func__.__closure__
# None
outer()().method.__func__.__closure__
# (<cell at 0xb7d655b4: type object at 0x82412bc>, <cell at 0xb7d655cc: int object at 0x81b20b0>)

我找不到太多关于在定义时如何处理它们的具体文档。上述解释正确吗？

原文

Specifically, how are free variables bound at definition for methods of a class? It is probably something like this:

enclosing function (temporary) scope => generate closure
global (permanent) scope => generate no closure (just look it up when the method body executes)
raise UnboundLocalError()

Here are two examples:

globalname = 0
class Test(object):
    def method(self):
        print globalname
        print Test

def outer():
    localname = 1
    class Test(object):
        def method(self):
            print globalname
            print localname
            print Test
    return Test

Test().method.__func__.__closure__
# None
outer()().method.__func__.__closure__
# (<cell at 0xb7d655b4: type object at 0x82412bc>, <cell at 0xb7d655cc: int object at 0x81b20b0>)

I couldn't find much documentation on specifically how they are treated at definition time. Is the above explanation correct?

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孤千羽 2024-09-19 09:01:26

当且仅当一个变量被分配到当前代码块内时，Python 才假定该变量是局部变量。因此，

spam = 0
def ham:
    print( spam )

将使垃圾邮件成为一个全局变量，但

spam = 0
def ham:
    spam = 0
    print( spam )

会成为一个单独的变量，对于ham来说是本地的。闭包获取封闭范围内的所有本地变量。在您的第一个示例中，没有局部变量，因此没有闭包；在第二个中，localname被分配，因此method是一个闭包。

与 Python 中一样，有很多方法可以绕过这个假设。 global 关键字将变量声明为全局（！），因此例如

spam = 0
def ham:
    global spam
    spam = 0
    print( spam )

将不是闭包。 Py3k 引入了 nonlocal 关键字，它告诉 Python 向上查找范围，直到找到变量名并引用它。

Python assumes a variable is local if and only if it is assigned to, within the current code block. So

spam = 0
def ham:
    print( spam )

will make spam a global variable, but

spam = 0
def ham:
    spam = 0
    print( spam )

will make a separate variable, local to ham. A closure grabs all the variables local to the enclosing scope. In your first example, there are no local variables so no closure; in the second, localname is assigned to and thus method is a closure.

As always in Python, there are ways around this assumption. The global keyword declares a variable as global (!), so e.g.

spam = 0
def ham:
    global spam
    spam = 0
    print( spam )

will not be a closure. Py3k introduces the nonlocal keyword, which tells Python to look upwards through the scopes until it finds the variable name and refer to that.

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