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