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参见：http://stackoverflow.com/questions/26595/is-there-any-difference-between-foo-is-none-and-foo-none
http://stackoverflow.com/questions/132988/is-there-a-difference-between-and-is-in-python?answertab=active#tab-top
Python中 “foo is None” 与“foo == None”的差别
原文地址：http://www.cnblogs.com/kelamoyujuzhen/p/5658436.html
