itertools.groupby() not grouping correctly

python itertools

user994165 · Nov 14, 2011 · Viewed 13.5k times · Source

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

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)):

itertools.groupby() not grouping correctly

Answer

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