python 2.7 - Finding a substring within a string without using any built in functions -

i have find particular substring within string , position within string. easy have without using built in functions or methods in python. appreciate if help. thanks

string="acaagatgccattgtcccccggcctcctgctgctgctgctctccggggccacggccaccgctgccctgcccctggagggtggccccaccggccgagacagcgagcatatgcaggaagcggcaggaataaggaaaagcagcctcctgactttcctcgcttggtggtttgagtggacctcccaggccagtgccgggcccctcataggagaggaagctcgggaggtggccaggcggcaggaaggcgcacccccccagcaatccgcgcgccgggacagaatgccctgcaggaacttcttctggaagaccttctcctcctgcaaataaaacctcacccatgaatgctcacgcaagtttaattacagacctgaa" 

substring='ttg'

this code work if don't want use inbuilt function.

man = "acaagatgccattgtcccccggcctcctgctgctgctgctctccggggccacggccaccgctgccctgcccc$ check = "ttg"  in range (0, len(man)):     if man[i]==check[0]:             sum =0             j in range(0, len(check)):                     if man[i+j] != check[j]:                             break                             sum = 0                     else:                             sum = sum +1                             if sum == len(check):                                 print