Petri Lehtinen <pe...@digip.org> added the comment:

You're right, the result over a set would be unpredictable.

The point of the itertools module is to be able to a) cope with massive amounts 
of data and b) be a set of tools instead of complete solutions for all problems.

Because of both of the points above, groupby() doesn't load all the data into 
memory or attempt to sort the data by itself. Furthermore, there's no way for 
groupby() to know whether the iterable it's passed is going to yield sorted or 
unsorted data.

It's your responsibility to know whether the iterable you're passing is already 
sorted or not, and sort it first, if it's possible and there's a need to do so.

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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue14828>
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