Python list of tuples: organize by unique elements to a dictionary

Question

I have a list of tuples, say:

list_of_tuples = [('a', 1),('b',2),('c',1),('a',2),('d',3)]

I need to get correspoding values for any (unique) second element in a tuple. For example as a dictionary. Output:

dict = {1:['a','c'],2:['b','a'],3:['d']}

Whats the most pythonic way of doing it? Help much appriciated!

Answer 1

I'd probably go with a defaultdict like jamylak, but if you want a "real" dictionary, you can use setdefault() :

>>> list_of_tuples = [('a', 1),('b',2),('c',1),('a',2),('d',3)]
>>> d = {}
>>> for item in list_of_tuples:
...     d.setdefault(item[1],[]).append(item[0])
...
>>> d
{1: ['a', 'c'], 2: ['b', 'a'], 3: ['d']}

Answer 2

>>> from collections import defaultdict
>>> list_of_tuples = [('a', 1),('b',2),('c',1),('a',2),('d',3)]
>>> d = defaultdict(list)
>>> for c,num in list_of_tuples:
        d[num].append(c)


>>> d
defaultdict(<type 'list'>, {1: ['a', 'c'], 2: ['b', 'a'], 3: ['d']})