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

This expands upon the answer by Robert, applying to when the values in the dict aren't unique.

class ReversibleDict(dict):

    def reversed(self):
        """
        Return a reversed dict, with common values in the original dict
        grouped into a list in the returned dict.

        Example:
        >>> d = ReversibleDict({'a': 3, 'c': 2, 'b': 2, 'e': 3, 'd': 1, 'f': 2})
        >>> d.reversed()
        {1: ['d'], 2: ['c', 'b', 'f'], 3: ['a', 'e']}
        """

        revdict = {}
        for k, v in self.iteritems():
            revdict.setdefault(v, []).append(k)
        return revdict

The implementation is limited in that you cannot use reversed twice and get the original back. It is not symmetric as such. It is tested with Python 2.6. Here is a use case of how I am using to print the resultant dict.

If you'd rather use a set than a list, and there could exist unordered applications for which this makes sense, instead of setdefault(v, []).append(k), use setdefault(v, set()).add(k).