On May 12, 2011, at 11:30 AM, Eric Snow wrote: > On Thu, May 12, 2011 at 6:23 AM, Roy Smith <r...@panix.com> wrote: > The docs say: > > [http://docs.python.org/library/stdtypes.html] > Objects of different types, except different numeric types and different > string types, never compare equal > > [http://docs.python.org/release/2.7/reference/expressions.html#notin] > objects of different types (emphasis)always compare unequal > > > That definitely makes it unclear.
I don't think it's unclear at all. It's very clear. Clearly wrong :-) > A little further down it says that you can customize comparison with the > __cmp__ special method. Yes, and that says: The operators <, >, ==, >=, <=, and != compare the values of two objects. The objects need not have the same type. If both are numbers, they are converted to a common type. Otherwise, objects of different types always compare unequal, and are ordered consistently but arbitrarily. You can control comparison behavior of objects of non-built-in types by defining a __cmp__ method or rich comparison methods like __gt__, described in section Special method names. I read that as saying that if you implement __eq__(), you must make sure that it returns False if self and other have different types (and likewise, __ne__() should return True for that case). The same way that it says that obj1.__lt__(obj2) must return a consistent result for all types of obj1 and obj2. > Looking over the documentation, it seems like it could be touched up to > alleviate any confusion. Perhaps rewording to make it clear that the > described behavior is the default for objects, rather than always the case. That would be a start, but doesn't address this specific problem. If the docs are changed to say that "type(obj1) != type(obj2) implies obj1 != obj2" in only the default, people will assume that list follows this default rule since it's a built-in type. Built-in types which don't follow the default rule need to have their behavior documented. --- Roy Smith r...@panix.com
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