Thought I would add this in case you are looking at how to do this for a technical interview where they don't want you to use Python's built-in function in or find, which is horrible, but does happen:
string = "Samantha"
word = "man"
def find_sub_string(word, string):
len_word = len(word) #returns 3
for i in range(len(string)-1):
if string[i: i + len_word] == word:
return True
else:
return False
