fix(db_api): allow file path for credentials (#221)

* fix: credentials_uri parameter error

* fix: combine credentials and credentials_uri

* fix: nits

Author: HemangChothani

google/cloud/spanner_dbapi/connection.py

@@ -357,10 +357,13 @@ def connect(
                     instances, tables and data. If not provided, will
                     attempt to determine from the environment.
 
-    :type credentials: :class:`~google.auth.credentials.Credentials`
+    :type credentials: Union[:class:`~google.auth.credentials.Credentials`, str]
     :param credentials: (Optional) The authorization credentials to attach to
                         requests. These credentials identify this application
-                        to the service. If none are specified, the client will
+                        to the service. These credentials may be specified as
+                        a file path indicating where to retrieve the service
+                        account JSON for the credentials to connect to
+                        Cloud Spanner. If none are specified, the client will
                         attempt to ascertain the credentials from the
                         environment.
 
@@ -384,9 +387,14 @@ def connect(
         user_agent=user_agent or DEFAULT_USER_AGENT, python_version=PY_VERSION
     )
 
-    client = spanner.Client(
-        project=project, credentials=credentials, client_info=client_info
-    )
+    if isinstance(credentials, str):
+        client = spanner.Client.from_service_account_json(
+            credentials, project=project, client_info=client_info
+        )
+    else:
+        client = spanner.Client(
+            project=project, credentials=credentials, client_info=client_info
+        )
 
     instance = client.instance(instance_id)
     if not instance.exists():

tests/unit/spanner_dbapi/test_connect.py

@@ -139,3 +139,28 @@ def test_sessions_pool(self):
             ):
                 connect("test-instance", database_id, pool=pool)
                 database_mock.assert_called_once_with(database_id, pool=pool)
+
+    def test_connect_w_credential_file_path(self):
+        from google.cloud.spanner_dbapi import connect
+        from google.cloud.spanner_dbapi import Connection
+
+        PROJECT = "test-project"
+        USER_AGENT = "user-agent"
+        credentials = "dummy/file/path.json"
+
+        with mock.patch(
+            "google.cloud.spanner_v1.Client.from_service_account_json"
+        ) as client_mock:
+            connection = connect(
+                "test-instance",
+                "test-database",
+                PROJECT,
+                credentials=credentials,
+                user_agent=USER_AGENT,
+            )
+
+            self.assertIsInstance(connection, Connection)
+
+            client_mock.assert_called_once_with(
+                credentials, project=PROJECT, client_info=mock.ANY
+            )