>>> from functools import lru_cache
>>> @lru_cache(maxsize=4095)
def ld(s, t):
	if not s: return len(t)
	if not t: return len(s)
	if s[0] == t[0]: return ld(s[1:], t[1:])
	l1 = ld(s, t[1:])
	l2 = ld(s[1:], t)
	l3 = ld(s[1:], t[1:])
	return 1 + min(l1, l2, l3)

>>> print( ld("kitten","sitting"),ld("rosettacode","raisethysword") )
3 8