feat(bigquery): add bigquery_federated_query_spanner sample to run a federated query against Cloud Spanner

Problem:
The BigQuery documentation (https://cloud.google.com/bigquery/docs/spanner-federated-queries)
needs a standalone, tested Python code sample demonstrating how to
run a federated query against Cloud Spanner (region tag `bigquery_federated_query_spanner`).

Solution:
- Add `federated_query_spanner(project_id: str, connection_id: str) -> None` in `bigquery/federated_queries/bigquery_federated_query_spanner.py`
  (Query a Cloud Spanner database table in real time from BigQuery using EXTERNAL_QUERY.)
- Add unit tests (`bigquery/federated_queries/bigquery_federated_query_spanner_test.py`) covering both the success
  path and API error handling (`GoogleAPICallError`)

Author: iennae

bigquery/federated_queries/bigquery_federated_query_spanner.py

@@ -0,0 +1,79 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# [START bigquery_federated_query_spanner]
+import sys
+
+from google.api_core import exceptions
+from google.cloud import bigquery
+
+
+def federated_query_spanner(
+    project_id: str,
+    connection_id: str,
+) -> None:
+    """Runs a federated query from BigQuery to Cloud Spanner using EXTERNAL_QUERY.
+
+    Args:
+        project_id: Google Cloud project ID (for example, 'example-project').
+        connection_id: Fully qualified BigQuery Cloud Spanner connection ID
+            (for example, 'example-project.us.example-spanner-conn').
+    """
+    client = bigquery.Client(project=project_id)
+    # Push down the order aggregation and filter to Cloud Spanner via EXTERNAL_QUERY,
+    # then join the summarized results with the public theLook eCommerce users table
+    # in bigquery-public-data (https://cloud.google.com/bigquery/public-data).
+    external_sql = (
+        "SELECT user_id, COUNT(order_id) AS completed_orders, "
+        "MIN(created_at) AS first_order_date "
+        "FROM orders WHERE status = 'Complete' GROUP BY user_id"
+    )
+    query = f"""
+        SELECT
+          u.id AS user_id,
+          u.country,
+          rq.completed_orders,
+          rq.first_order_date
+        FROM EXTERNAL_QUERY('{connection_id}', "{external_sql}") AS rq
+        INNER JOIN `bigquery-public-data.thelook_ecommerce.users` AS u
+          ON u.id = rq.user_id
+        ORDER BY user_id ASC
+        LIMIT 10;
+    """
+
+    try:
+        print(f"Running Spanner federated query via connection: {connection_id}")
+        query_job = client.query(query)
+        rows = list(query_job.result())
+    except exceptions.NotFound as err:
+        print(
+            f"error: Spanner connection '{connection_id}' not found in project "
+            f"'{project_id}': {err.message}",
+            file=sys.stderr,
+        )
+        return
+    except exceptions.GoogleAPICallError as err:
+        print(
+            f"error: could not run Spanner federated query on connection "
+            f"'{connection_id}' in project '{project_id}': {err.message}",
+            file=sys.stderr,
+        )
+        return
+
+    print(f"Resource ID: {connection_id}")
+    print(f"Job ID: {query_job.job_id}")
+    print(f"Rows Returned: {len(rows)}")
+
+
+# [END bigquery_federated_query_spanner]

bigquery/federated_queries/bigquery_federated_query_spanner_test.py

@@ -0,0 +1,70 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from unittest import mock
+
+import bigquery_federated_query_spanner
+from google.api_core import exceptions
+import pytest
+
+
+def test_federated_query_spanner_success(
+    monkeypatch: pytest.MonkeyPatch,
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    mock_client = mock.MagicMock()
+    mock_job = mock.MagicMock()
+    mock_job.job_id = "example-job-123"
+    mock_row = mock.MagicMock()
+    mock_job.result.return_value = [mock_row]
+    mock_client.query.return_value = mock_job
+    monkeypatch.setattr(
+        bigquery_federated_query_spanner.bigquery,
+        "Client",
+        lambda project=None: mock_client,
+    )
+
+    bigquery_federated_query_spanner.federated_query_spanner(
+        project_id="example-project",
+        connection_id="example-project.us.example-spanner-conn",
+    )
+
+    captured = capsys.readouterr()
+    assert "Resource ID:" in captured.out
+    assert "example-project" in captured.out
+    assert mock_client.query.called
+
+
+def test_federated_query_spanner_error(
+    monkeypatch: pytest.MonkeyPatch,
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    mock_client = mock.MagicMock()
+    mock_job = mock.MagicMock()
+    mock_job.result.side_effect = exceptions.NotFound("resource error")
+    mock_client.query.return_value = mock_job
+    monkeypatch.setattr(
+        bigquery_federated_query_spanner.bigquery,
+        "Client",
+        lambda project=None: mock_client,
+    )
+
+    bigquery_federated_query_spanner.federated_query_spanner(
+        project_id="example-project",
+        connection_id="example-project.us.example-spanner-conn",
+    )
+
+    captured = capsys.readouterr()
+    assert "error:" in captured.err
+    assert "example-project" in captured.err

bigquery/federated_queries/noxfile_config.py

@@ -0,0 +1,23 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+TEST_CONFIG_OVERRIDE = {
+    # You can opt out from the test for specific Python versions.
+    "ignored_versions": ["2.7", "3.7", "3.8", "3.9", "3.10", "3.11"],
+    # Enforce type hints on new samples.
+    "enforce_type_hints": True,
+    "gcloud_project_env": "GOOGLE_CLOUD_PROJECT",
+    "pip_version_override": None,
+    "envs": {},
+}

bigquery/federated_queries/requirements-test.txt

@@ -0,0 +1 @@
+pytest==9.0.3; python_version >= "3.10"

bigquery/federated_queries/requirements.txt

@@ -0,0 +1 @@
+google-cloud-bigquery==3.47.0