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

I would like to give another example where groupby without sort is not working. Adapted from example by James Sulak

from itertools import groupby

things = [("vehicle", "bear"), ("animal", "duck"), ("animal", "cactus"), ("vehicle", "speed boat"), ("vehicle", "school bus")]

for key, group in groupby(things, lambda x: x[0]):
    for thing in group:
        print "A %s is a %s." % (thing[1], key)
    print " "

output is

A bear is a vehicle.

A duck is a animal.
A cactus is a animal.

A speed boat is a vehicle.
A school bus is a vehicle.

there are two groups with vehicule, whereas one could expect only one group