Classes as dictionary keys in Python

Question

So, I've done a lot of research on this and saw various other links and conferred with the Python documentation, however, I'm still a bit unclear on this.

Maybe the way I see classes in Python is a bit wrong.

As I understand, keys in a dictionary must be immutable. However, classes can be keys in a dictionary because of their default hash implementation (I think?). Why is this the case since classes are mutable?

For example,

class C:
    def __init__(self):
        self.val = 15
        self.array = []

c = C()

D = {c: 15}
c.val = 14
c.array.append(15)

print(D[c])

Why is this okay?

Answer 1

Instances of your C class are actually hashable, it comes with a default implementation of __hash__ which pertains to the identity of that object:

>>> hash(c) # calls c.__hash__()
306169194

This __hash__ implementation allows your instance to be used as a key in a dictionary.

This explains "Why doesn't changing things in your class change the hash?" — because the identity/reference of the instance doesn't change even if its contents do.

On older versions of python, this used to be exactly equal to the object's id, but from python 3 onwards, it seems to be some derivative of it. This post goes into the gory details on the hash implementation.

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Now let's say you wanted to prevent instances of your class from being used as a key... you could then do this (from the documentation):

If a class that does not override __eq__() wishes to suppress hash support, it should include __hash__ = None in the class definition.

class C:
    def __init__(self):
        self.val = 15
        self.array = []

    __hash__ = None # suppressing hash support

c = C()

And now you get a familiar sounding TypeError if you attempt to retrieve the hash of c:

>>> hash(c)
# TypeError: unhashable type: 'C'

Naturally, this also implies you cannot use c as a dictionary key anymore (IOW trying to initialize {c: 15} would also throw the same TypeError since c is not hashable).