use defaultdict

[CatUnderTheLeaf]

check code. I think there are lots of places where this can be used instead

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

sorted(d.items())
[('blue', [2, 4]), ('red', [1]), ('yellow', [1, 3])]

or

s = [('red', 1), ('blue', 2), ('red', 3), ('blue', 4), ('red', 1), ('blue', 4)]
d = defaultdict(set)
for k, v in s:
d[k].add(v)

sorted(d.items())
[('blue', {2, 4}), ('red', {1, 3})]