python - Converting a list of tuples into a dict

Question

I have a list of tuples like this:

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

I want to iterate through this keying by the first item, so, for example, I could print something like this:

a 1 2 3
b 1 2
c 1

How would I go about doing this without keeping an item to track whether the first item is the same as I loop around the tuples? This feels rather messy (plus I have to sort the list to start with)...

Answer 1

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

d = {}
for x, y in l:
    d.setdefault(x, []).append(y)
print d

produces:

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