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The Python docs clearly state that x==y calls x.__eq__(y) . However it seems that under many circumstances, the opposite is true. Where is it documented when or why this happens, and how can I work out for sure whether my object's __cmp__ or __eq__ methods are going to get called. Edit: Just to clarify, I know that __eq__ is called in preferecne to __cmp__ , but I'm not clear why y.__eq__(x) is called in preference to x.__eq__(y) , when the latter is what the docs state will happen. >>> class TestCmp(object): ... def __cmp__(self, other): ... print "__cmp__ got called" ... return 0 ... >>> class TestEq(object): ... def __eq__(self, other): ... print "__eq__ got called" ... return True ... >>> tc = TestCmp() >>> te = TestEq() >>> >>> 1 == tc __cmp__ got called True >>> tc == 1 __cmp__ got called True >>> >>> 1 == te __eq__ got called True >>> te == 1 __eq__ got called True >>> >>> class TestStrCmp(str): ... def __new__(cls, value): ... return str.__new__(cls, value) ... ... def __cmp__(self, other): ... print "__cmp__ got called" ... return 0 ... >>> class TestStrEq(str): ... def __new__(cls, value): ... return str.__new__(cls, value) ... ... def __eq__(self, other): ... print "__eq__ got called" ... return True ... >>> tsc = TestStrCmp("a") >>> tse = TestStrEq("a") >>> >>> "b" == tsc False >>> tsc == "b" False >>> >>> "b" == tse __eq__ got called True >>> tse == "b" __eq__ got called True Edit: From Mark Dickinson's answer and comment it would appear that: Rich comparison overrides __cmp__</code

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