def nth_split(s, delim, n):
    p, c = -1, 0
while c < n:
    p = s.index(delim, p + 1)
c += 1
return s[:p], s[p + 1:]
s1, s2 = nth_split('20_231_myString_234', '_', 2)
print(s1, ':', s2)
Split string at nth occurrence of a given character