code: def pystr(self, min_chars=None, max_chars=20):
          
          if min_chars is None:
              return "".join(self.random_letters(length=max_chars))
          else:
              assert (
                  max_chars >= min_chars), "Maximum length must be greater than or equal to minium length"
              return "".join(
                  self.random_letters(
                      length=self.generator.random.randint(min_chars, max_chars),
                  ),
              )
docs: Generates a random string of upper and lowercase letters.
          :type min_chars: int
          :type max_chars: int
          :return: String. Random of random length between min and max characters.