为什么在这种情况下不需要全局关键字？ [英] Why is the global keyword not required in this case?

问题描述

cache = {}
def func():
    cache['foo'] = 'bar'
print cache['foo'] 

输出

output

bar

为什么这个工作以及为什么它不需要使用 global 关键字

Why does this work and why doesn't it require use of the global keyword?

推荐答案

因为您不是赋值到 cache ，您改变字典本身。 cache 仍然指向字典，因此本身不变。 cache ['foo'] ='bar'行转换为 cache .__ setitem __（'foo'，'bar'）。换句话说， cache 的值是一个python dict ，并且该值本身是可变的。

Because you are not assigning to cache, you are changing the dictionary itself instead. cache is still pointing to the dictionary, thus is itself unchanged. The line cache['foo'] = 'bar' translates to cache.__setitem__('foo', 'bar'). In other words, the value of cache is a python dict, and that value is itself mutable.

如果您试图通过使用 cache ='bar' cache $ c>，而是改变 cache 点数，然后你需要全局关键字。

If you tried to change what cache refers to by using cache = 'bar' instead, you would be changing what cache points to and then you need the global keyword.

也许这个类似问题的这个较老的答案可以帮助你理解不同之处： Python列表不反映变量变化。

Perhaps this older answer of mine to a similar question helps you understand the difference: Python list doesn't reflect variable change.

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