lcsm problem on Rosalind

lcsm.py

#!/usr/bin/env python

import fastaparse
import sys

sequences = [i.seq for i in fastaparse.parse_fasta(sys.argv[1])]

def substrings(string):
	n = len(string)
	for length in range(n,0,-1):
		for offset in range(n-length):
			yield(string[offset:offset+length])

first = sequences.pop()
sub = substrings(first)

for motif in sub:
	if all((motif in seq for seq in sequences)):
		break
print(motif)