return the cost (in GB) if dry run is set to True

Author: antoineeripret

pandas_gbq/gbq_connector.py

@@ -270,7 +270,7 @@ def run_query(
         dtypes = kwargs.get("dtypes")
 
         if dry_run:
-            return rows_iter
+            return rows_iter.total_bytes_processed / 1024**3
 
         return self._download_results(
             rows_iter,

tests/system/test_gbq.py

@@ -658,16 +658,16 @@ def test_columns_and_col_order_raises_error(self, project_id):
 
     def test_read_gbq_with_dry_run(self, project_id):
         query = "SELECT 1"
-        job = gbq.read_gbq(
+        cost = gbq.read_gbq(
             query,
             project_id=project_id,
             credentials=self.credentials,
             dialect="standard",
             dry_run=True,
         )
-        assert job.dry_run
-        assert job.state == "DONE"
-        assert job.total_bytes_processed > 0
+        assert isinstance(cost, float)
+        assert cost > 0
+
 
 
 class TestToGBQIntegration(object):

tests/unit/test_gbq.py

@@ -939,8 +939,10 @@ def test_run_query_with_dml_query(mock_bigquery_client, mock_query_job):
     mock_bigquery_client.list_rows.assert_not_called()
 
 
-def test_read_gbq_with_dry_run(mock_bigquery_client):
-    gbq.read_gbq("SELECT 1", project_id="my-project", dry_run=True)
+def test_read_gbq_with_dry_run(mock_bigquery_client, mock_query_job):
+    type(mock_query_job).total_bytes_processed = mock.PropertyMock(return_value=12345)
+    cost = gbq.read_gbq("SELECT 1", project_id="my-project", dry_run=True)
     _, kwargs = mock_bigquery_client.query.call_args
     job_config = kwargs["job_config"]
     assert job_config.dry_run is True
+    assert cost == 12345 / 1024**3