How can I group equivalent items together in a Python list?

Question

I have a list like

x = [2, 2, 1, 1, 1, 1, 1, 1]

I would like to put the repeated numbers together like

[[2,2],[1,1,1,1,1,1]]

What about `[2,2,1,1,1,2,2]`? Should it be `[[2,2],[1,1,1],[2,2]]` or `[[2,2,2,2],[1,1,1]]`? — Michał Bentkowski, Aug 11 '11 at 12:11

agf · Answer 1 · 2011-08-11T12:15:54.087

27

[list(g) for k, g in itertools.groupby(iterable)]

This is exactly what itertools.groupby is for.

If you want nonconsecutive numbers grouped, like in the comment by @Michal,

[list(g) for k, g in itertools.groupby(sorted(iterable))]

edited Aug 11 '11 at 12:15

answered Aug 11 '11 at 12:10

agf

Nice trick. What would this solution work for? I mean iterables of ints, floats, strings I guess it's ok. How about iterables of your own class objects ? Would these work if __eq__ or __hash__ is implemented ? – Bogdan Aug 11 '11 at 12:27
1

So long as they compare equal it will work; code equivalent to the implementation is at the link in my answer. So, just implement `__eq__` (and also `__hash__` because objects that are equal need to hash the same) and it will work. – agf Aug 11 '11 at 12:30
3

You don't need `__hash__` just for this code to work, but it's a good idea to implement it so that your objects can be used effectively in dicts and sets. – Ned Batchelder Aug 11 '11 at 12:38

1 Answers1