How to use a dictionary as key for a dictionary in Python

Question

All -

I have a very basic question today ... but it has been preventing me from moving on productively in my programming, so I am posting it here.

I want to create a dictionary that takes a dictionary as a key. Presumably I can only pass a reference to the dictionary as key ... only I don't know how to do that in Python. Here is a toy reduction of what I am trying to do:

def test( dict ):
    a={}
    b={1:1}
    a[ dict ] = b
    return a

a = {0:0}
print test( a ) 

I would like b to be a dictionary of the form { {0:0} : {1:1} }.

Any help with this is much appreciated.

Kind regards -

Pat

Answer 1

Keys for dictionaries must be hashable items; unfortunately, dictionaries themselves are not hashable (they are mutable, which disqualifies them from being hashable).

Use their item list, converted to a sorted tuple, instead:

a[tuple(sorted(dct.items()))] = b

Mutable objects are not hashable because they can be changed in-place, making later key-lookups fail. What would the expected outcome be if you added or removed items from the dictionary you used as the key, for example?

Answer 2

Dictionaries can only use hashable objects as keys, which means they must be immutable. The default dictionary isn't either, so that won't work. You can try a frozendict though.