input: Write a function to take a list of strings and print the longest string in the list.
  
  def print_longest_string(str_list):
      longest_str = ''
      for str in str_list:
          if len(str) > len(longest_str):
              longest_str = str
      
      print(longest_str)
  
  assert print_longest_string(['foo', 'test', 'longstring', 'bar'])=='longstring'
  assert print_longest_string(['short', 'longerstring', 'medium'])=='longerstring'
  assert print_longest_string(['a', 'b', 'c'])=='a'
  assert print_longest_string(['word', 'longer', 'shortest'])=='longer'
  assert print_longest_string([])=='None'
output: def print_longest_string(str_list):
      longest_str = ''
      for str in str_list:
          if len(str) > len(longest_str):
              longest_str = str
      
      print(longest_str)