input: def longest_string(str_list): 
      """
      Find and return the longest string in a provided list of strings.
      
      >>> longest_string(['foo', 'test', 'longstring', 'bar'])
      'longstring'
      >>> longest_string(['a', 'ab', 'abc', 'abcd'])
      'abcd'
      """
      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