Sorting sub-lists into new sub-lists based on common first items

Question

I have a large number of two-membered sub-lists that are members of a list called mylist:

mylist = [[\'AB001\', 22100],
          [\'AB001\', 32         


        

Answer 1

collections.defaultdict

An itertools.groupby solution will incur O(n log n) cost since the input must be sorted first. You can a use defaultdict of lists for a guaranteed O(n) solution:

from collections import defaultdict

dd = defaultdict(list)
for item in mylist:
    dd[item[0]].append(item)

res = list(dd.values())

print(res)

[[['AB001', 22100], ['AB001', 32935], ['AB001', 34439]],
 [['XC013', 99834], ['XC013', 86701]],
 [['VD126', 18884]]]