Python: create a list of list of lists

You can do this using two applications of itertools.groupby, one to group by ID, and one to group by date.

The code below uses a triple-nested list comprehension, which is compact, but not so easy to read. I’ll post a longer version shortly.

from itertools import groupby
from operator import itemgetter

data=""'\
ID  date    product
A   01/01/2018  1
A   01/01/2018  2
A   02/01/2018  2
B   01/01/2018  3
B   01/01/2018  4
B   02/01/2018  2
B   04/01/2018  1
B   04/01/2018  2
B   04/01/2018  3
'''

data = (row.split() for row in data.splitlines())

#skip header
next(data)

result = [[[u[-1] for u in group]
    for _, group in groupby(row, itemgetter(1))]
        for _, row in groupby(data, itemgetter(0))]

print(result)

output

[[['1', '2'], ['2']], [['3', '4'], ['2'], ['1', '2', '3']]]

---

Here’s a version (mostly) using traditional for loops. It also converts the product numbers from string to integer.

from itertools import groupby
from operator import itemgetter

data=""'\
ID  date    product
A   01/01/2018  1
A   01/01/2018  2
A   02/01/2018  2
B   01/01/2018  3
B   01/01/2018  4
B   02/01/2018  2
B   04/01/2018  1
B   04/01/2018  2
B   04/01/2018  3
'''

data = (row.split() for row in data.splitlines())

#skip header
next(data)

ig1 = itemgetter(1)
result = []
for _, row in groupby(data, itemgetter(0)):
    sublist = []
    for _, group in groupby(row, ig1):
       sublist.append([int(u[-1]) for u in group])
    result.append(sublist)

print(result)

output

[[[1, 2], [2]], [[3, 4], [2], [1, 2, 3]]]