I have a feeling someone asked this question very recently, but I can't find it now. Assuming that the dropped letters were an accident, couldn't you just do:
def split_at_idx(s, idx):
return [s[i:j] for i,j in zip([0]+idx, idx+[None])]
after which we have
>>> split_at_idx(string.ascii_letters, [3, 10, 12, 40])
['abc', 'defghij', 'kl', 'mnopqrstuvwxyzABCDEFGHIJKLMN', 'OPQRSTUVWXYZ']
>>> split_at_idx(string.ascii_letters, [])
['abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ']
>>> split_at_idx(string.ascii_letters, [13, 26, 39])
['abcdefghijklm', 'nopqrstuvwxyz', 'ABCDEFGHIJKLM', 'NOPQRSTUVWXYZ']