[Python-Dev] Documentation Error for hash (original) (raw)

Jesse Noller jnoller at gmail.com
Sat Aug 30 21:13:32 CEST 2008

On Aug 30, 2008, at 2:41 PM, "Ondrej Certik" <ondrej at certik.cz> 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 tphash slot to PyObjectHashNotImplemented - using one of the above approaches will cause hash(obj) to raise a TypeError, as will calling the tphash 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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