Python 全局变量/范围混淆 [英] Python global variable/scope confusion

问题描述

我已经开始自学 Python，并且注意到全局变量和作用域有些奇怪.当我运行这个:

I've started teaching myself python, and have noticed something strange to do with global variables and scope. When I run this:

x = 2 
y = 3 
z=17 
def add_nums():
    y = 6 
    return z+y

打印出23的结果...但是，当我将回报扩大为:

The result of 23 is printed... However, when I expand the return to be:

x = 2 
y = 3 
z=17 
def add_nums(): 
    y = 6 
    z = z + y 
    return z

我在第 6 行收到以下错误:

I get the following error on line 6:

Local name referenced but not bound to a value.
A local name was used before it was created. You need to define the     
method or variable before you try to use it.

<小时>

我很困惑为什么我在这里收到错误，因为 z 是全局可访问的.

---

I'm confused as why I am getting an error here, as z is global an accessible.

推荐答案

当一个变量在等号的左边时，python 会创建一个局部变量.当变量位于等号的右侧时，python 将尝试查找局部变量，如果找不到，它将使用全局变量.在您的示例中， z 位于等号的右侧和左侧，以避免歧义 python 引发错误.您需要使用 global 语法来避免这种情况:

When a variable is on the left side of an equal sign python will create a local variable. When the variable is on the right side of the equal sign python will try to look for a local variable and if he can't find one then it will use a global variable. In your example, z is on the right and left side of the equal sign, to avoid ambiguities python raises an error. You need to use the global syntax to avoid this:

x = 2 
y = 3 
z=17 
def add_nums(): 
    global z
    y = 6 
    z = z + y 
    return z

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