Immutable dictionary, only use as a key for another dictionary

Question

I had the need to implement a hashable dict so I could use a dictionary as a key for another dictionary.

A few months ago I used this implementation: Python hashable

Answer 1

It appears I am late to post. Not sure if anyone else has come up with ideas. But here is my take on it. The Dict is immutable and hashable. I made it immutable by overriding all the methods, magic and otherwise, with a custom '_readonly' function that raises an Exception. This is done when the object is instantiated. To get around the problem of not being able to apply the values I set the 'hash' under '__new__'. I then I override the '__hash__'function. Thats it!

class ImmutableDict(dict):

_HASH = None

def __new__(cls, *args, **kwargs):
    ImmutableDict._HASH = hash(frozenset(args[0].items()))
    return super(ImmutableDict, cls).__new__(cls, args)

def __hash__(self):
    return self._HASH

def _readonly(self, *args, **kwards):
    raise TypeError("Cannot modify Immutable Instance")

__delattr__ = __setattr__ = __setitem__ = pop = update = setdefault = clear = popitem = _readonly

Test:

immutabled1 = ImmutableDict({"This": "That", "Cheese": "Blarg"})

dict1 = {immutabled1: "Yay"}

dict1[immutabled1]

"Yay"

dict1

{{'Cheese': 'Blarg', 'This': 'That'}: 'Yay'}