if char in s:
    c = char[0]
for ch in s:
    if ch == c:
        if s[index:index + len(char)] == char:
            return index
index += 1
return -1
print(find_str('Happy birthday', 'py'))
print(find_str('Happy birthday', 'rth'))
print(find_str('Happy birthday', 'rh'))
Python: Find a substring in a string and returning the index of the substring