Is there a hash of a class instance in Python?

Question

Let’s suppose I have a class like this:

class MyClass:
    def __init__(self, a):
        self._a = a

And I construct such instances:

obj1 = MyClass(5)
obj2 = MyClass(12)
obj3 = MyClass(5)

Is there a general way to hash my objects such that objects constructed with same values have equal hashes? In this case:

myhash(obj1) != myhash(obj2)
myhash(obj1) == myhash(obj3)

By general I mean a Python function that can work with objects created by any class I can define. For different classes and same values the hash function must return different results, of course; otherwise this question would be about hashing of several arguments instead.

Asked By: Fomalhaut

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Source

Answer 1

def myhash(obj):
    items = sorted(obj.__dict__.items(), key=lambda it: it[0])
    return hash((type(obj),) + tuple(items))

This solution obviously has limitations:

It assumes that all fields in __dict__ are important.
It assumes that __dict__ is present, e.g. this won’t work with __slots__.
It assumes that all values are hashable
It breaks the Liskov substitution principle.

Answered By: Alex Hall

Answer 2

The question is badly formed for a couple reasons:

Hashes don’t test eqaulity, just inequality. That is, they guarantee that hash(a) != hash(b) implies a != b, but the reverse does not hold true. For example, checking "aKey" in myDict will do a linear search through all keys in myDict that have the same hash as "aKey".
You seem to wanting to do something with storage. Note that the hash of "aKey" will change between runs, so don’t write it to a file. See the bottom of __hash__ for more information.
In general, you need to think carefully about subclasses, hashes, and equality. There is a pit here, so even the official documentation quietly sidesteps what the hash of instance means. Do note that each instance has a __dict__ for local variables and the __class__ with more information.

Hope this helps those who come after you.

Answered By: Charles Merriam

Is there a hash of a class instance in Python?

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