code: def next_string(min_size, max_size):
          
          result = ''
          
          max_size = max_size if max_size != None else min_size
          length = RandomInteger.next_integer(min_size, max_size)
          for i in range(length):
              result += random.choice(_chars)
  
          return result
docs: Generates a random string, consisting of upper and lower case letters (of the English alphabet),
          digits (0-9), and symbols ("_,.:-/.[].{},#-!,$=%.+^.&*-() ").
  
          :param min_size: (optional) minimum string length.
  
          :param max_size: maximum string length.
  
          :return: a random string.