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Add warning about subclassing behavior (#485) 

Fixes #483
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Hynek Schlawack 2019-02-05 06:45:27 +01:00 committed by GitHub
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@ -37,6 +37,16 @@ Because according to the definition_ from the official Python docs, the returned
The *correct way* to achieve hashing by id is to set ``@attr.s(cmp=False)``. The *correct way* to achieve hashing by id is to set ``@attr.s(cmp=False)``.
Setting ``@attr.s(hash=False)`` (that implies ``cmp=True``) is almost certainly a *bug*. Setting ``@attr.s(hash=False)`` (that implies ``cmp=True``) is almost certainly a *bug*.
.. warning::
Be careful when subclassing!
Setting ``cmp=False`` on class whose base class has a non-default ``__hash__`` method, will *not* make ``attrs`` remove that ``__hash__`` for you.
It is part of ``attrs``'s philosophy to only *add* to classes so you have the freedom to customize your classes as you wish.
So if you want to *get rid* of methods, you'll have to do it by hand.
The easiest way to reset ``__hash__`` on a class is adding ``__hash__ = object.__hash__`` in the class body.
#. If two object are not equal, their hash **should** be different. #. If two object are not equal, their hash **should** be different.
While this isn't a requirement from a standpoint of correctness, sets and dicts become less effective if there are a lot of identical hashes. While this isn't a requirement from a standpoint of correctness, sets and dicts become less effective if there are a lot of identical hashes.