On Aug 30, 2008, at 2:41 PM, "Ondrej Certik" <[EMAIL PROTECTED]> wrote:
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
Ondrej, a patch that improves the official docs would be welcome and
still potentially make 2.6/3.0
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