[ENH] pull in warning for dialect change from pandas-gbq.

Author: tswast

doc/source/whatsnew/v0.24.0.txt

@@ -170,8 +170,8 @@ Other Enhancements
 - :meth:`Series.droplevel` and :meth:`DataFrame.droplevel` are now implemented (:issue:`20342`)
 - Added support for reading from Google Cloud Storage via the ``gcsfs`` library (:issue:`19454`)
 - :func:`to_gbq` and :func:`read_gbq` signature and documentation updated to
-  reflect changes from the `Pandas-GBQ library version 0.5.0
-  <https://pandas-gbq.readthedocs.io/en/latest/changelog.html#changelog-0-5-0>`__.
+  reflect changes from the `Pandas-GBQ library version 0.6.0
+  <https://pandas-gbq.readthedocs.io/en/latest/changelog.html#changelog-0-6-0>`__.
   (:issue:`21627`)
 - New method :meth:`HDFStore.walk` will recursively walk the group hierarchy of an HDF5 file (:issue:`10932`)
 - :func:`read_html` copies cell data across ``colspan`` and ``rowspan``, and it treats all-``th`` table rows as headers if ``header`` kwarg is not given and there is no ``thead`` (:issue:`17054`)

pandas/io/gbq.py

@@ -1,5 +1,7 @@
 """ Google BigQuery support """
 
+import warnings
+
 
 def _try_import():
     # since pandas is a dependency of pandas-gbq
@@ -23,7 +25,7 @@ def _try_import():
 
 def read_gbq(query, project_id=None, index_col=None, col_order=None,
              reauth=False, private_key=None, auth_local_webserver=False,
-             dialect='legacy', location=None, configuration=None,
+             dialect=None, location=None, configuration=None,
              verbose=None):
     """
     Load data from Google BigQuery.
@@ -65,6 +67,8 @@ def read_gbq(query, project_id=None, index_col=None, col_order=None,
 
         *New in version 0.2.0 of pandas-gbq*.
     dialect : str, default 'legacy'
+        Note: The default value is changing to 'standard' in a future verion.
+
         SQL syntax dialect to use. Value can be one of:
 
         ``'legacy'``
@@ -108,6 +112,17 @@ def read_gbq(query, project_id=None, index_col=None, col_order=None,
     pandas.DataFrame.to_gbq : Write a DataFrame to Google BigQuery.
     """
     pandas_gbq = _try_import()
+
+    if dialect is None:
+        dialect = "legacy"
+        warnings.warn(
+            'The default value for dialect is changing to "standard" in a '
+            'future version of pandas-gbq. Pass in dialect="legacy" to '
+            "disable this warning.",
+            FutureWarning,
+            stacklevel=2,
+        )
+
     return pandas_gbq.read_gbq(
         query, project_id=project_id, index_col=index_col,
         col_order=col_order, reauth=reauth, verbose=verbose,