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

Function is symmetric for values of type list; Tuples are coverted to lists when performing reverse_dict(reverse_dict(dictionary))

def reverse_dict(dictionary):
    reverse_dict = {}
    for key, value in dictionary.iteritems():
        if not isinstance(value, (list, tuple)):
            value = [value]
        for val in value:
            reverse_dict[val] = reverse_dict.get(val, [])
            reverse_dict[val].append(key)
    for key, value in reverse_dict.iteritems():
        if len(value) == 1:
            reverse_dict[key] = value[0]
    return reverse_dict