Is there any reason *not* to cache an object's hash?

user_76701 0
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I've written a class whose .__hash__() implementation takes a long time to execute. I've been thinking to cache its hash, and store it in a variable like ._hash so the .__hash__() method would simply return ._hash. (Which will be computed either at the end of the .__init__() or the first time .__hash__() is called.)

My reasoning was: "This object is immutable -> Its hash will never change -> I can cache the hash."

But now that got me thinking: You can say the same thing about any hashable object. (With the exception of objects whose hash is their id.)

So is there ever a reason not to cache an object's hash, except for small objects whose hash computation is very fast?

hash caching python
3 Answers

hash(obj) == id(obj) does not hold in general:

    >>> class A(object): pass
    ... 
    >>> a = A()
    >>> hash(a), id(a)
    (8762051845337, 140192829525392)
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