Accessing Firestore document in Firebase emulator fails with AttributeError

[nksma]

The recent change (4381ad5) assumes _credentials to have id_token attribute, but that's not the case all the time.

https://github.com/googleapis/python-firestore/blame/main/google/cloud/firestore_v1/base_client.py#L183

When it doesn't contain the id_token attribute, AttributeError is raised.
Before checking if id_token is None, we should check if the attribute exists using hasattr()

Environment details

• OS type and version: Mac OS X (Darwin mac-mini.lan 20.5.0)
• Python version: 3.8.8
• pip version: 21.1.2
• google-cloud-firestore version: 2.3.0

Steps to reproduce

1. run unit test that reads and writes Firestore document using firebase-emulator:exec

Code example

omitted since it's a straightforward bug.

Stack trace

  File "/opt/venv/lib/python3.8/site-packages/google/cloud/firestore_v1/document.py", line 167, in set
     write_results = batch.commit(**kwargs)
  File "/opt/venv/lib/python3.8/site-packages/google/cloud/firestore_v1/batch.py", line 59, in commit
    commit_response = self._client._firestore_api.commit(
   File "/opt/venv/lib/python3.8/site-packages/google/cloud/firestore_v1/client.py", line 109, in _firestore_api
    return self._firestore_api_helper(
   File "/opt/venv/lib/python3.8/site-packages/google/cloud/firestore_v1/base_client.py", line 152, in _firestore_api_helper
     channel = self._emulator_channel(transport)
   File "/opt/venv/lib/python3.8/site-packages/google/cloud/firestore_v1/base_client.py", line 183, in _emulator_channel
     if self._credentials is not None and self._credentials.id_token is not None:
 AttributeError: 'Credentials' object has no attribute 'id_token'

[crwilcox]

What credentials object are you creating? In our testing this wasn't seen, so I wonder if you are creating a non default one? We can harden this, but I am still curious.

[nksma]

This is how I'm creating the credentials.
cred = firebase_admin.credentials.ApplicationDefault()
firebase_admin.initialize_app(cred)

Hope this helps with improving the test coverage to avoid a similar issue in the future.