[TST] Add test for BigQuery dialect change warning

pandas/tests/io/test_gbq.py

@@ -4,6 +4,11 @@
 import platform
 import os
 
+try:
+    from unittest import mock
+except ImportError:
+    mock = pytest.importorskip("mock")
+
 import numpy as np
 import pandas as pd
 from pandas import compat, DataFrame
@@ -93,6 +98,14 @@ def make_mixed_dataframe_v2(test_size):
                      index=range(test_size))
 
 
+def test_read_gbq_without_dialect_warns_future_change(monkeypatch):
+    mock_read_gbq = mock.Mock()
+    mock_read_gbq.return_value = DataFrame([[1.0]])
+    monkeypatch.setattr(pandas_gbq, 'read_gbq', mock_read_gbq)
+    with pytest.warns(FutureWarning):
+        pd.read_gbq("SELECT 1")
+
+
 @pytest.mark.single
 class TestToGBQIntegrationWithServiceAccountKeyPath(object):