Ethan Furman added the comment: So something like: For container types such as list, tuple, or collections.deque, the expression 'x in y' is equivalent to 'any(x is e or x == e for e in y)'. For container types such as set, frozenset, and dict, this equivalence expression is modified by the addition of 'if hash(x) == hash(e)'. ? ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue23987> _______________________________________