首次使用后重新分配时，局部变量出现 UnboundLocalError [英] UnboundLocalError on local variable when reassigned after first use

问题描述

以下代码在 Python 2.5 和 3.0 中均按预期工作:

The following code works as expected in both Python 2.5 and 3.0:

a, b, c = (1, 2, 3)

print(a, b, c)

def test():
    print(a)
    print(b)
    print(c)    # (A)
    #c+=1       # (B)
test()

但是，当我取消注释 (B) 行时，我在 (A) 行收到 UnboundLocalError: 'c' not assignment.a 和 b 的值打印正确.这让我完全困惑，原因有两个:

However, when I uncomment line (B), I get an UnboundLocalError: 'c' not assigned at line (A). The values of a and b are printed correctly. This has me completely baffled for two reasons:

1. 为什么会因为 (B) 行后面的语句而在 (A) 行抛出运行时错误?

1. Why is there a runtime error thrown at line (A) because of a later statement on line (B)?

为什么变量 a 和 b 按预期打印，而 c 会引发错误?

Why are variables a and b printed as expected, while c raises an error?

我能想到的唯一解释是local变量c是由赋值c+=1创建的，它优先甚至在创建局部变量之前就覆盖了全局"变量 c.当然，在变量存在之前窃取"作用域是没有意义的.

The only explanation I can come up with is that a local variable c is created by the assignment c+=1, which takes precedent over the "global" variable c even before the local variable is created. Of course, it doesn't make sense for a variable to "steal" scope before it exists.

有人可以解释一下这种行为吗?

Could someone please explain this behavior?

推荐答案

Python 以不同的方式处理函数中的变量，具体取决于您是从函数内部还是外部为其赋值.如果在函数内分配了变量，则默认情况下将其视为局部变量.因此，当您取消注释该行时，您试图在为其分配任何值之前引用局部变量 c.

Python treats variables in functions differently depending on whether you assign values to them from inside or outside the function. If a variable is assigned within a function, it is treated by default as a local variable. Therefore, when you uncomment the line you are trying to reference the local variable c before any value has been assigned to it.

如果想让变量c引用函数前赋值的全局c=3，把

If you want the variable c to refer to the global c = 3 assigned before the function, put

global c

作为函数的第一行.

至于 python 3，现在有

As for python 3, there is now

nonlocal c

您可以使用它来引用最近的具有 c 变量的封闭函数作用域.

that you can use to refer to the nearest enclosing function scope that has a c variable.

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