How should we use global variables correctly?

[Windson Yang]

I can 'feel' that global variables are evil. I also read lots of articles
proves that (http://wiki.c2.com/?GlobalVariablesAreBad). However, I found
CPython Lib use quite a lot of `global` keyword. So how should we use
`global` keyword correctly? IIUC, it's fine that we use `global` keyword
inside the Lib since most of the time the user just import the lib and call
the API. In any other situation, we should avoid using it. Am I right?
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