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57 строки
3.3 KiB
ReStructuredText
57 строки
3.3 KiB
ReStructuredText
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Hashing
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=======
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.. warning::
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The overarching theme is to never set the ``@attr.s(hash=X)`` parameter yourself.
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Leave it at ``None`` which means that ``attrs`` will do the right thing for you, depending on the other parameters:
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- If you want to make objects hashable by value: use ``@attr.s(frozen=True)``.
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- If you want hashing and comparison by object identity: use ``@attr.s(cmp=False)``
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Setting ``hash`` yourself can have unexpected consequences so we recommend to tinker with it only if you know exactly what you're doing.
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Under certain circumstances, it's necessary for objects to be *hashable*.
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For example if you want to put them into a :class:`set` or if you want to use them as keys in a :class:`dict`.
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The *hash* of an object is an integer that represents the contents of an object.
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It can be obtained by calling :func:`hash` on an object and is implemented by writing a ``__hash__`` method for your class.
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``attrs`` will happily write a ``__hash__`` method you [#fn1]_, however it will *not* do so by default.
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Because according to the definition_ from the official Python docs, the returned hash has to fullfil certain constraints:
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#. Two objects that are equal, **must** have the same hash.
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This means that if ``x == y``, it *must* follow that ``hash(x) == hash(y)``.
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By default, Python classes are compared *and* hashed by their :func:`id`.
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That means that every instance of a class has a different hash, no matter what attributes it carries.
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It follows that the moment you (or ``attrs``) change the way equality is handled by implementing ``__eq__`` which is based on attribute values, this constraint is broken.
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For that reason Python 3 will make a class that has customized equality unhashable.
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Python 2 on the other hand will happily let you shoot your foot off.
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Unfortunately ``attrs`` currently mimics Python 2's behavior for backward compatibility reasons if you set ``hash=False``.
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The *correct way* to achieve hashing by id is to set ``@attr.s(cmp=False)``.
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Setting ``@attr.s(hash=False)`` (that implies ``cmp=True``) is almost certainly a *bug*.
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#. If two object are not equal, their hash **should** be different.
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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.
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The worst case is when all objects have the same hash which turns a set into a list.
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#. The hash of an object **must not** change.
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If you create a class with ``@attr.s(frozen=True)`` this is fullfilled by definition, therefore ``attrs`` will write a ``__hash__`` function for you automatically.
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You can also force it to write one with ``hash=True`` but then it's *your* responsibility to make sure that the object is not mutated.
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This point is the reason why mutable structures like lists, dictionaries, or sets aren't hashable while immutable ones like tuples or frozensets are:
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point 1 and 2 require that the hash changes with the contents but point 3 forbids it.
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For a more thorough explanation of this topic, please refer to this blog post: `Python Hashes and Equality`_.
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.. [#fn1] The hash is computed by hashing a tuple that consists of an unique id for the class plus all attribute values.
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.. _definition: https://docs.python.org/3/glossary.html#term-hashable
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.. _`Python Hashes and Equality`: https://hynek.me/articles/hashes-and-equality/
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