Python类中的全局变量和局部变量

x = "xtop"
y = "ytop"
def func():
    x = "xlocal"
    y = "ylocal"
    class C:
        print x  #xlocal  of course
        print y  #ytop  why? I guess output may be 'ylocal' or '1'
        y = 1
        print y  #1  of course
func()

Why x and y are different here？

为什么x和y在这里有所不同?
If I replace class C with a function scope I will get UnboundLocalError: local variable 'y' referenced before assignment，What is the difference between a class and a function in this situation?

如果我用函数作用域替换C类,我将得到UnboundLocalError:在赋值之前引用局部变量'y',在这种情况下,类和函数之间有什么区别?

1 个解决方案

#1

The reason for this is because the scope of class C is actually different than the scope of def func - and the different defaulting behaviours of scopes that python has.

原因是因为C类的范围实际上与def函数的范围不同 - 以及python具有的范围的不同默认行为。

Here is basically how python looks for a variable

这里基本上是python如何查找变量

Look in current scope

查看当前范围

If current scope doesn't have it -> use nearest enclosing scope

如果当前范围没有 - >使用最近的封闭范围

If current scope has it, but not yet defined -> use global scope

如果当前范围有,但尚未定义 - >使用全局范围

If current scope has it, and already defined -> use it

如果当前范围有,并且已经定义 - >使用它

else blow up

否则爆炸

(If you remove ytop you get a NameError: name 'y' is not defined)

(如果删除ytop,则会出现NameError:未定义名称'y')

So basically, when the interpreter looks at the following section of code it goes

所以基本上,当解释器查看下面的代码部分时,它就会出现

class C:
    print(x) # need x, current scope no x  -> default to nearest (xlocal)
    print(y) # need y, current scope yes y -> default to global (ytop)
             #         but not yet defined 
    y = 1
    print(y) # okay we have local now, switch from global to local

Consider the following scenarios

请考虑以下方案

1) class C:
    print(x)
    print(y)

>>> xlocal
>>> ylocal

2) class C:
    y = 1
    print(x)
    print(y)  

>>> xlocal
>>> 1

3) class C:
    print(x)
    print(y)
    x = 1

>>> xtop
>>> ylocal

#1