Reverse / invert a dictionary mapping

Question

Given a dictionary like so:

my_map = {\'a\': 1, \'b\': 2}

How can one invert this map to get:

inv_map = {1: \'a\', 2: \'b\'         


        

Answer 1

Fast functional solution for non-bijective maps (values not unique):

from itertools import imap, groupby

def fst(s):
    return s[0]

def snd(s):
    return s[1]

def inverseDict(d):
    """
    input d: a -> b
    output : b -> set(a)
    """
    return {
        v : set(imap(fst, kv_iter))
        for (v, kv_iter) in groupby(
            sorted(d.iteritems(),
                   key=snd),
            key=snd
        )
    }

In theory this should be faster than adding to the set (or appending to the list) one by one like in the imperative solution.

Unfortunately the values have to be sortable, the sorting is required by groupby.