[Python-Dev] Documentation Error for __hash__

[Ondrej Certik]

> I strongly advise people interested in this topic to take a closer look
> at the functionality that was added to address issue 2235. The "don't
> inherit __hash__" behaviour has been backported from 3.0 and broke a
> bunch of code, but the naive fix of reverting to the 2.5 behaviour
> proceeded to make the 2.6 version of collections.Hashable completely
> useless (since pretty much *everything* then claimed to be hashable,
> including all of the container types in the standard library).
>
> After thrashing out those problems, the 2.6 behaviour ended up being:
> - __hash__ is still inherited by default
> - you can block inheritance explicitly by setting "__hash__ = None" in
> the class definition or on the class object
> - for a class defined in C, you can block __hash__ inheritance by
> setting the tp_hash slot to PyObject_HashNotImplemented
> - using one of the above approaches will cause hash(obj) to raise a
> TypeError, as will calling the tp_hash slot directly
> - unlike defining your own __hash__ method, using one of the above
> techniques will also mean that isinstance(obj, collections.Hashable)
> will also return False
> - defining __eq__ and/or __cmp__ without defining __hash__ will lead to
> a Py3k Warning being raised on the class definition when run with the -3
> switch
>
> The Python 3.0 behaviour is similar to the above, except that the
> unconditional inheritance of __hash__ is gone. If you define __eq__ or
> __cmp__ without defining __hash__, __hash__ will NOT be inherited from
> the parent class - instead, it will be set to None.
>
> The documentation of all of this could definitely use a bit more work -
> that's why I reopened 2235 in response to Michael's post.

Couple months ago I was trying to figure out how __eq__, __cmp__,
__hash__ and similar methods are called in Python and in which order
and wrote a docs here:

http://docs.sympy.org/python-comparisons.html

Feel free to reuse any of it. I wrote it to myself, as I didn't find
the official docs very clear.

Ondrej