feat: Ensure service account file exists before initializing credentials

pyfcm/baseapi.py

@@ -8,7 +8,7 @@
 import requests
 from requests.adapters import HTTPAdapter
 from urllib3 import Retry
-
+from os import path
 from google.oauth2 import service_account
 from google.oauth2.credentials import Credentials
 import google.auth.transport.requests
@@ -177,6 +177,10 @@ def _initialize_credentials(self):
         Initialize credentials and FCM endpoint if not already initialized.
         """
         if self.credentials is None:
+            if not path.isfile(self._service_account_file):
+                raise InvalidDataError(f"The service account file you passed does not exist at '{self._service_account_file}'. "
+                                        "Ensure it does not have any typos and exists."
+                                        )
             self.credentials = service_account.Credentials.from_service_account_file(
                 self._service_account_file,
                 scopes=["https://www.googleapis.com/auth/firebase.messaging"],

tests/test_fcm.py

@@ -8,6 +8,33 @@ def test_push_service_without_credentials():
     except errors.AuthenticationError:
         pass
 
+def test_push_service_with_incorrect_service_account_file():
+    try:
+        fcm = FCMNotification(service_account_file='./foo.json', project_id=None, credentials=None)
+        fcm.notify()
+        assert False, "Should raise InvalidDataError without correct service account file path"
+    except errors.InvalidDataError:
+        pass
+
+def test_push_service_with_valid_service_account_file(mocker):
+    # When the service account file exists, credentials must be built via
+    # google.oauth2.service_account.Credentials.from_service_account_file.
+    # google.oauth2.credentials.Credentials does not provide that method.
+    mocker.patch("pyfcm.baseapi.path.isfile", return_value=True)
+    mock_from_file = mocker.patch(
+        "pyfcm.baseapi.service_account.Credentials.from_service_account_file",
+        return_value="dummy-credentials",
+    )
+
+    fcm = FCMNotification(
+        service_account_file="./service_account.json",
+        project_id="test",
+        credentials=None,
+    )
+    fcm._initialize_credentials()
+
+    mock_from_file.assert_called_once()
+    assert fcm.credentials == "dummy-credentials"
 
 def test_push_service_directly_passed_credentials(push_service):
     # We should infer the project ID/endpoint from credentials