itertools.groupby() not grouping correctly

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)]
         


        

Answer 1

Below "fixes" several annoyances with Python's itertools.groupby.

def groupby2(l, key=lambda x:x, val=lambda x:x, agg=lambda x:x, sort=True):
    if sort:
        l = sorted(l, key=key)
    return ((k, agg((val(x) for x in v))) \
        for k,v in itertools.groupby(l, key=key))

Specifically,

1. It doesn't require that you sort your data.
2. It doesn't require that you must use key as named parameter only.
3. The output is clean generator of tuple(key, grouped_values) where values are specified by 3rd parameter.
4. Ability to apply aggregation functions like sum or avg easily.

Example Usage

import itertools
from operator import itemgetter
from statistics import *

t = [('a',1), ('b',2), ('a',3)]
for k,v in groupby2(t, itemgetter(0), itemgetter(1), sum):
  print(k, v)

This prints,

a 4
b 2

Play with this code