register fails when using client_secret

[jkoscialkowski]

It seems impossible to register a user when using a client secret.

Steps to reproduce: have a user pool with no users and with an app client with a secret.

u = Cognito(user_pool_id='eu-central-1_...',
    client_id='...',
    client_secret='...')
u.set_base_attributes(email='person@example.com')
u.register('person', 'password')

Crashes with:

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-22-d291537f4dbd> in <module>
      1 u.set_base_attributes(email='jan.koscialkowski@gmail.com')
----> 2 u.register('person', '...')

~/.local/lib/python3.8/site-packages/pycognito/__init__.py in register(self, username, password, attr_map)
    346             "UserAttributes": cognito_attributes,
    347         }
--> 348         self._add_secret_hash(params, "SecretHash")
    349         response = self.client.sign_up(**params)
    350 

~/.local/lib/python3.8/site-packages/pycognito/__init__.py in _add_secret_hash(self, parameters, key)
    690         """
    691         if self.client_secret is not None:
--> 692             secret_hash = AWSSRP.get_secret_hash(
    693                 self.username, self.client_id, self.client_secret
    694             )

~/.local/lib/python3.8/site-packages/pycognito/aws_srp.py in get_secret_hash(username, client_id, client_secret)
    199     @staticmethod
    200     def get_secret_hash(username, client_id, client_secret):
--> 201         message = bytearray(username + client_id, "utf-8")
    202         hmac_obj = hmac.new(bytearray(client_secret, "utf-8"), message, hashlib.sha256)
    203         return base64.standard_b64encode(hmac_obj.digest()).decode("utf-8")

TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'

This is caused by https://github.com/pvizeli/pycognito/blob/6c173cf0bb26e40a941be188670c7c592b46eeb8/pycognito/__init__.py#L693 operating on self.username which is not initialized.
I think adding self.username = username around https://github.com/pvizeli/pycognito/blob/6c173cf0bb26e40a941be188670c7c592b46eeb8/pycognito/__init__.py#L334 would fix it without breaking any code using _add_secret_hash. Please let me know if this is OK for you and I will submit a PR.

[gavihs]

I am having the same issue

[dleber]

I found I can work around this by providing the username when instantiating the class, i.e.

u = Cognito(USER_POOL_ID, CLIENT_ID, client_secret=CLIENT_SECRET, username=my_username)