input: Write a python function to find the longest string between two given strings.
  assert longest_string("Python", "programming") == "programming"
  assert longest_string("hello", "world") == "hello"
  assert longest_string("short", "longer") == "longer"
output: def longest_string(str1, str2):
      return str1 if len(str1) > len(str2) else str2