Question

I have this data:

self.data = [(1, 1, 5.0),
             (1, 2, 3.0),
             (1, 3, 4.0),
             (2, 1, 4.0),
             (2, 2, 2.0)]

When I run this code:

for mid, group in itertools.groupby(self.data, key=operator.itemgetter(0)):

for list(group) I get:

[(1, 1, 5.0),
 (1, 2, 3.0),
 (1, 3, 4.0)]

which is what I want.

But if I use 1 instead of 0

for mid, group in itertools.groupby(self.data, key=operator.itemgetter(1)):

to group by the second number in the tuples, I only get:

[(1, 1, 5.0)]

even though there are other tuples that have “1” in that 1 (2nd) position.

Answer 1

itertools.groupby collects together contiguous items with the same key.
If you want all items with the same key, you have to sort self.data first.

for mid, group in itertools.groupby(
    sorted(self.data,key=operator.itemgetter(1)), key=operator.itemgetter(1)):