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 using non-nested dictionaries as keys:

frozenset(a.items())#Can be put in the dictionary instead

Is there a method of using arbitrary objects as keys in dictionaries?

Example:

How would this be used as a key?

{"a":1, "b":{"c":10}}

It is extremely rare that you will actually have to use something like this in your code. If you think this is the case, consider changing your data model first.

Exact use case

The use case is caching calls to an arbitrary keyword only function. Each key in the dictionary is a string (the name of the argument) and the objects can be quite complicated, consisting of layered dictionaries, lists, tuples, ect.

Related problems

This sub-problem has been split off from the problem here. Solutions here deal with the case where the dictionaries is not layered.

Answer 1

Based off solution by Chris Lutz again.

import collections

def hashable(obj):
    if isinstance(obj, collections.Hashable):
        items = obj
    elif isinstance(obj, collections.Mapping):
        items = frozenset((k, hashable(v)) for k, v in obj.iteritems())
    elif isinstance(obj, collections.Iterable):
        items = tuple(hashable(item) for item in obj)
    else:
        raise TypeError(type(obj))

    return items