Python string substr

The task is to find a the first string as a substring in the other string.
For example, let's say we have:
string 1 = 'bob'
string 2 = 'bobsbugsbegone'
for i in s2:
s1 == to [b]ONLY[/b] [0:2] of s2

I want to write a code that detects when there's a substring in a string and that counts how many substrings are found. (the second part is easy)

This is my code:

 

import sys

 

s1 = sys.argv[1]

s2 = sys.argv[2]

k = 0

i = 0

 

def substrings(s1, s2, k):

    for k in s2:

        for n in range(len(s2)):

            if k == s1[n]:

                return True

                print('s1 is a substring in s2')

            else:

                print('s1 is not a substring in s2')

                return False

         

print(substrings(s1, s2, k))

Help?

In addition to that:
In the first try it works
In the second try it also works
In the third try, i found out that it works only if the substring is at the beginning of the second string