Python: split list of strings to a list of lists of strings by length with a nested comprehensions

Question

I've got a list of strings and I'm trying to make a list of lists of strings by string length.

i.e.

['a', 'b', 'ab', 'abc'] 

becomes

[['a', 'b'], ['ab'], ['abc']]

I've accomplished this like so:

lst = ['a', 'b', 'ab', 'abc']
lsts = []
for num in set(len(i) for i in lst):
    lsts.append([w for w in lst if len(w) == num])

I'm fine with that code, but I'm trying to wrap my head around comprehensions. I want to use nested comprehensions to do the same thing, but I can't figure out how.

Answer 1

>>> [[w for w in L if len(w) == num] for num in set(len(i) for i in L)]
[['a', 'b'], ['ab'], ['abc']]

Also, itertools is likely to be a little more efficient.

Answer 2

lst = ['a', 'b', 'ab', 'abc']
lst.sort(key=len) # does not make any change on this data,but
                  # all strings of given length must occur together


from itertools import groupby
lst = [list(grp) for i,grp in groupby(lst, key=len)]

results in

[['a', 'b'], ['ab'], ['abc']]