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]}