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pythonpython-3.xpython-descriptors

Why is `__dict__` attribute of a custom Python class instance a descriptor of the class, instead of an actual attribute of the instances?

From https://stackoverflow.com/a/44880260/156458

Note that the __dict__ attribute of custom Python class instances is a descriptor; the instance itself doesn't have the attribute, it is the class that provides it (so type(instance).__dict__['__dict__'].__get__(instance) is returned). object.__dict__ may exist, but object.__dict__['__dict__'] does not.

Why is __dict__ attribute of a custom Python class instance a descriptor of the class, instead of an actual attribute of the instance?

Solution

  • It's tempting to say that __dict__ has to be a descriptor because implementing it as a __dict__ entry would require you to find the __dict__ before you can find the __dict__, but Python already bypasses normal attribute lookup to find __dict__ when looking up other attributes, so that's not quite as compelling as it initially sounds. If the descriptors were replaced with a '__dict__' key in every __dict__, __dict__ would still be findable.

    There's some space savings by not having a key for '__dict__' in every __dict__, but that's not the big reason. There's also time saved by not having to set a '__dict__' key, and time and space saved by not creating a circular reference, and these benefits are all really nice, but they're still probably smaller than the next thing.

    The big thing requiring __dict__ to be a descriptor is handling attempts to reassign or delete an object's __dict__. If attribute lookup for __dict__ went through a __dict__ key, then reassigning someobj.__dict__ would reassign the dict key without changing what dict Python actually looks in to find someobj's attributes. __dict__ needs to be a descriptor so it stays in sync with the actual C-level struct slot Python looks in to find an object's dict.