How To Split A Sorted List By Element Length

I have a list of lists and it is sorted by the length of the children lists. e.g.

[[str], [str1, str2], [str1, str2], [str1, str2, str3], [str1, str2, str3],...]

I would like to split this list into sublists that only contain the children with the same length. e.g.

[[[str], [str], [str]],  [[str1, str2], [str1, str2], [str1, str2]], ...]

I was wondering if there is a more efficient approach than mine below with hopefully a little less code.

child_list = []
new_list = []
old_list = [['e3510000'], ['e2512001'], ['e3510000'], ['e92d4010'],
            ['e3a0b000', 'e3a0e000'], ['e92d4030', 'e59f5054'],
            ['e59f3038', 'e3530000'], ['e1a0c00d', 'e92dd800']]

# length of child
length = 1
for idx, i in enumerate(old_list):
    if idx == len(old_list)-1:
        child_list.append(i)
        new_list.append(child_list.copy())
    elif length == len(i):
        child_list.append(i)
    elif length < len(i):
        new_list.append(child_list.copy())
        del child_list[:]
        child_list.append(i)
        length = len(i)

Output:

[[['e3510000'], ['e2512001'], ['e3510000'], ['e92d4010']],
 [['e3a0b000', 'e3a0e000'], ['e92d4030', 'e59f5054'], 
  ['e59f3038', 'e3530000'], ['e1a0c00d', 'e92dd800']]]

You can use itertools.groupby in order to group the lists in old by length. Note that if the original lists are already ordered according to their length as in your example, sorting here is not necessary.

from itertools import groupby
[list(v) for k,v in groupby(sorted(old_list, key=len), key=len)]

Output

[[['e3510000'], ['e2512001'], ['e3510000'], ['e92d4010']],
 [['e3a0b000', 'e3a0e000'],
  ['e92d4030', 'e59f5054'],
  ['e59f3038', 'e3530000'],
  ['e1a0c00d', 'e92dd800']]]