How do I use itertools.groupby()?

Question

I haven\'t been able to find an understandable explanation of how to actually use Python\'s itertools.groupby() function. What I\'m trying to do is this:

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Answer 1

A neato trick with groupby is to run length encoding in one line:

[(c,len(list(cgen))) for c,cgen in groupby(some_string)]

will give you a list of 2-tuples where the first element is the char and the 2nd is the number of repetitions.

Edit: Note that this is what separates itertools.groupby from the SQL GROUP BY semantics: itertools doesn't (and in general can't) sort the iterator in advance, so groups with the same "key" aren't merged.