Using non-hashable Python objects as keys in dictionaries

Question

Python doesn\'t allow non-hashable objects to be used as keys in other dictionaries. As pointed out by Andrey Vlasovskikh, there is a nice workaround for the special case of

Answer 1

With recursion!

def make_hashable(h):
    items = h.items()
    for item in items:
        if type(items) == dict:
            item = make_hashable(item)
    return frozenset(items)

You can add other type tests for any other mutable types you want to make hashable. It shouldn't be hard.

Answer 2

I agree with Lennart Regebro that you don't. However I often find it useful to cache some function calls, callable object and/or Flyweight objects, since they may use keyword arguments.

But if you really want it, try pickle.dumps (or cPickle if python 2.6) as a quick and dirty hack. It is much faster than any of the answers that uses recursive calls to make items immutable, and strings are hashable.

import pickle
hashable_str = pickle.dumps(unhashable_object)