a = "GAGCCTACTAACGGGAT" b = "CATCGTAATGACGGCCT" def distance_fnc(s1, s2): if len(s1) == len(s2): return len(s1) - len(list(filter(None, map(lambda v: v[0] == v[1], zip(s1, s2))))) else: return None def distance(x, y): d = 0 if len(x) != len(y): return None for i, c in enumerate(x): if c != y[i]: d += 1 return d print(distance(a, b) == distance_fnc(a, b))