substring.py

def isSubstring(substring, string):
    """
    substring: string
    string: string
    returns true if substring is substring of string e.g. 'cat' is substring of 'something cat something'
    """
    si = 0 # substring indexer
    for i in range(len(string)):
        if string[i] == substring[si]:
            si += 1 # notice, advancing the substring indexer each time we find a match
            if si == (len(substring)): # if the substring indexer reaches the length of the substring, it is a substring
                return True
        else: # also notice, we reset the substring indexer if we don't find a match, thus, only consecutive matches count
            si = 0
    return False
  
s = 'something'
s2 = 'cat'
phrase = 'once uppon a time something happened'
  
print(isSubstring(s, phrase))
print(isSubstring(s2, phrase))