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

I want to sort mylist into new sub-lists based on whether the sub-lists contain the same string as the first item. For example, this is what I am looking for my code to output:

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

Here is how I was trying to code this:

mylist = sorted(mylist)
newlist = []
for sublist in mylist:
    id = sublist[0]
if id == next.id:
    newlist.append(id)
print newlist

I was also trying to understand if itertools.groupby() was the correct tool for this problem. Can someone help me with this problem?

Answer 1

You were right about this being a job for groupby:

from itertools import groupby
from operator import itemgetter

mylist = [['AB001', 22100],
          ['AB001', 32935],
          ['XC013', 99834],
          ['VD126', 18884],
          ['AB001', 4439],
          ['XC013', 86701]]

print([list(value) for key, value in groupby(sorted(mylist), key=itemgetter(0))])

This will give you a list-of-lists, grouped by the first item in the sublist.

[[['AB001', 4439], ['AB001', 22100], ['AB001', 32935]], 
 [['VD126', 18884]], 
 [['XC013', 86701], ['XC013', 99834]]]