Question

To illustrate, I start with a list of 2-tuples:

import itertools
import operator

raw = [(1, "one"),
       (2, "two"),
       (1, "one"),
       (3, "three"),
       (2, "two")]

for key, grp in itertools.groupby(raw, key=lambda item: item[0]):
    print key, list(grp).pop()[1]

yields:

1 one
2 two
1 one
3 three
2 two

In an attempt to investigate why:

for key, grp in itertools.groupby(raw, key=lambda item: item[0]):
    print key, list(grp)

# ---- OUTPUT ----
1 [(1, 'one')]
2 [(2, 'two')]
1 [(1, 'one')]
3 [(3, 'three')]
2 [(2, 'two')]

Even this will give me the same output:

for key, grp in itertools.groupby(raw, key=operator.itemgetter(0)):
    print key, list(grp)

I want to get something like:

1 one, one
2 two, two
3 three

I am thinking this is because the key is within the tuple inside the list, when in fact the tuple gets moved around as one. Is there a way to get to my desired output? Maybe groupby() isn’t suited for this task?

Answer 1

groupby clusters consecutive elements of the iterable which have the same key.
To produce the output you desire, you must first sort raw.

for key, grp in itertools.groupby(sorted(raw), key=operator.itemgetter(0)):
    print key, map(operator.itemgetter(1), grp)

# 1 ['one', 'one']
# 2 ['two', 'two']
# 3 ['three']