Python defaultdict

>>> s = [('yellow', 1), ('blue', 2), ('yellow', 3), ('blue', 4), ('red', 1)]  
>>> d = defaultdict(list)  
>>> for k, v in s:
...             d[k].append(v)
...  
>>> list(d.items())
 [('blue', [2, 4]), ('red', [1]), ('yellow', [1, 3])]