# Functions are First-Class Citizens in Python 一等公民
https://cn.bing.com/search?form=MOZSBR&pc=MOZI&q=python++function++First+class+citizens
Python札记8:什么是first-class function? - 知乎 https://zhuanlan.zhihu.com/p/60754224


​co_freevars​

tuple of names of free variables (referenced via a function’s closure)