Immutable dictionary, only use as a key for another dictionary

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 dicts However I got a notice from a colleague saying 'it is not really immutable, thus it is not safe. You can use it, but it does make me feel like a sad Panda'. So I started looking around to create one that is immutable. I have no