code: def code_verifier(n_bytes=64):
      
      verifier = base64.urlsafe_b64encode(os.urandom(n_bytes)).rstrip(b'=')
      # https://tools.ietf.org/html/rfc7636#section-4.1
      # minimum length of 43 characters and a maximum length of 128 characters.
      if len(verifier) < 43:
          raise ValueError("Verifier too short. n_bytes must be > 30.")
      elif len(verifier) > 128:
          raise ValueError("Verifier too long. n_bytes must be < 97.")
      else:
          return verifier
docs: Generates a 'code_verifier' as described in section 4.1 of RFC 7636.
  
      This is a 'high-entropy cryptographic random string' that will be
      impractical for an attacker to guess.
  
      Args:
          n_bytes: integer between 31 and 96, inclusive. default: 64
              number of bytes of entropy to include in verifier.
  
      Returns:
          Bytestring, representing urlsafe base64-encoded random data.