Is there a 'multimap' implementation in Python?

Question

I am new to Python, and I am familiar with implementations of Multimaps in other languages. Does Python have such a data structure built-in, or available in a commonly-used

Answer 1

Or subclass dict:

class Multimap(dict):
    def __setitem__(self, key, value):
        if key not in self:
            dict.__setitem__(self, key, [value])  # call super method to avoid recursion
        else
            self[key].append(value)

Answer 2

I do not clearly understand the semantics of your example

a[1] = 'a'
a[1] = 'b' #??

Is second line a[1] = 'b' supposed to replace the element at [1]. If yes, then you need to use dictionary. If not - you need to use dictionary of lists (as already suggested)