docs: def rand_str(length, allowed=CHARSET_ALPHA_DIGITS):
      """Generate fixed-length random string from your allowed character pool.
  
      :param length: total length of this string.
      :param allowed: allowed charset.
  
      Example::
  
          >>> import string
          >>> rand_str(32)
          H6ExQPNLzb4Vp3YZtfpyzLNPFwdfnwz6
      """
      res = list()
      for _ in range(length):
          res.append(random.choice(allowed))
      return "".join(res)