everything in python is object
assignment is binding a name to an object
one object can have several binded name
changes to any binded name will reflect on every binding
https://www.jeffknupp.com/blog/2013/02/14/drastically-improve-your-python-understanding-pythons-execution-model/
原文：http://www.cnblogs.com/lynnge/p/5077228.html