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?