input: def longest_string(str_list):
      """ Find and return the longest string in a given list of strings.
      >>> longest_string(['foo', 'test', 'longstring', 'bar'])
      'longstring'
      """
      longest_str = ''
      for str in str_list:
          if len(str) > len(longest_str):
              longest_str = str
    
      return longest_str
output:     longest_str = ''
      for str in str_list:
          if len(str) > len(longest_str):
              longest_str = str
    
      return longest_str