[Python-Dev] Documentation Error for hash (original) (raw)
Nick Coghlan ncoghlan at gmail.com
Fri Aug 29 18:04:52 CEST 2008
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Matt Giuca wrote:
It's probably a good idea to implement hash for objects that implement comparisons, but it won't always work and it is certainly not needed, unless you intend to use them as dictionary keys.
So you're suggesting that we document something like. Classes that represent mutable values and define equality methods are free to define hash so long as you don't mind them being used incorrectly if treated as dictionary keys... Technically true, but not very helpful in my opinion... :-) No, I think he was suggesting we document that if a class overrides eq, it's a good idea to also implement hash, so it can be used as a dictionary key.
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.
Cheers, Nick.
-- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia
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