feat: Support passing google.auth typed credentials in initialize_app() (#821)

* feat: Support passing `google.auth` typed credentials in `initialize_app()`

* Refactor and add unit test

Author: jonathanedey

firebase_admin/__init__.py

@@ -18,6 +18,7 @@
 import os
 import threading
 
+from google.auth.credentials import Credentials as GoogleAuthCredentials
 from google.auth.exceptions import DefaultCredentialsError
 from firebase_admin import credentials
 from firebase_admin.__about__ import __version__
@@ -208,10 +209,13 @@ def __init__(self, name, credential, options):
                              'non-empty string.'.format(name))
         self._name = name
 
-        if not isinstance(credential, credentials.Base):
+        if isinstance(credential, GoogleAuthCredentials):
+            self._credential = credentials._ExternalCredentials(credential) # pylint: disable=protected-access
+        elif isinstance(credential, credentials.Base):
+            self._credential = credential
+        else:
             raise ValueError('Illegal Firebase credential provided. App must be initialized '
                              'with a valid credential instance.')
-        self._credential = credential
         self._options = _AppOptions(options)
         self._lock = threading.RLock()
         self._services = {}

firebase_admin/credentials.py

@@ -18,6 +18,7 @@
 import pathlib
 
 import google.auth
+from google.auth.credentials import Credentials as GoogleAuthCredentials
 from google.auth.transport import requests
 from google.oauth2 import credentials
 from google.oauth2 import service_account
@@ -58,6 +59,19 @@ def get_credential(self):
         """Returns the Google credential instance used for authentication."""
         raise NotImplementedError
 
+class _ExternalCredentials(Base):
+    """A wrapper for google.auth.credentials.Credentials typed credential instances"""
+
+    def __init__(self, credential: GoogleAuthCredentials):
+        super(_ExternalCredentials, self).__init__()
+        self._g_credential = credential
+
+    def get_credential(self):
+        """Returns the underlying Google Credential
+
+        Returns:
+          google.auth.credentials.Credentials: A Google Auth credential instance."""
+        return self._g_credential
 
 class Certificate(Base):
     """A credential initialized from a JSON certificate keyfile."""

tests/test_app.py

@@ -246,6 +246,16 @@ def test_non_default_app_init(self, app_credential):
         with pytest.raises(ValueError):
             firebase_admin.initialize_app(app_credential, name='myApp')
 
+    def test_app_init_with_google_auth_cred(self):
+        cred = testutils.MockGoogleCredential()
+        assert isinstance(cred, credentials.GoogleAuthCredentials)
+        app = firebase_admin.initialize_app(cred)
+        assert cred is app.credential.get_credential()
+        assert isinstance(app.credential, credentials.Base)
+        assert isinstance(app.credential, credentials._ExternalCredentials)
+        with pytest.raises(ValueError):
+            firebase_admin.initialize_app(app_credential)
+
     @pytest.mark.parametrize('cred', invalid_credentials)
     def test_app_init_with_invalid_credential(self, cred):
         with pytest.raises(ValueError):