不明白为什么会出现UnboundLocalError(闭包) [英] Don't understand why UnboundLocalError occurs (closure)

问题描述

我在这里做错了什么?

counter = 0

def increment():
  counter += 1

increment()

上面的代码抛出一个UnboundLocalError.

推荐答案

Python 没有变量声明，所以它必须弄清楚 范围 变量本身.它通过一个简单的规则来实现:如果在函数内部对变量进行赋值，则该变量被视为本地变量.^[1] 因此，该行

Python doesn't have variable declarations, so it has to figure out the scope of variables itself. It does so by a simple rule: If there is an assignment to a variable inside a function, that variable is considered local.^[1] Thus, the line

counter += 1

隐式地使 counter 局部于 increment().但是，尝试执行此行将尝试在分配之前读取局部变量 counter 的值，从而导致 UnboundLocalError.^[2]

implicitly makes counter local to increment(). Trying to execute this line, though, will try to read the value of the local variable counter before it is assigned, resulting in an UnboundLocalError.^[2]

如果 counter 是一个全局变量，global 关键字会有所帮助.如果 increment() 是局部函数而 counter 是局部变量，则可以使用 nonlocal.

If counter is a global variable, the global keyword will help. If increment() is a local function and counter a local variable, you can use nonlocal in Python 3.x.

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