add testcase

Author: shuoweil

tests/system/small/test_anywidget.py

@@ -13,6 +13,8 @@
 # limitations under the License.
 
 
+import unittest.mock as mock
+
 import pandas as pd
 import pytest
 
@@ -684,6 +686,23 @@ def test_widget_with_unknown_row_count_empty_dataframe(
         assert widget.page == 0
 
 
-# TODO(shuowei): Add tests for custom index and multiindex
+def test_repr_html_anywidget_fallback(paginated_bf_df: bf.dataframe.DataFrame):
+    """
+    Test that _repr_html_ falls back to deferred mode when anywidget is not available.
+    """
+    with bf.option_context("display.repr_mode", "anywidget"):
+        # Use a mock to simulate the absence of the 'anywidget' module.
+        with mock.patch.dict(
+            "sys.modules", {"anywidget": None, "IPython": mock.MagicMock()}
+        ):
+            # The warning is now expected inside the _ipython_display_ call, not _repr_html_
+            # The test setup doesn't easily allow capturing warnings from ipython display hooks.
+            # Instead we focus on the fallback behavior of _repr_html_
+            html = paginated_bf_df._repr_html_()
+            assert "Computation deferred." in html
+            assert "Computation will process" in html
+
+
+# TODO(b/332316283): Add tests for custom index and multiindex
 # This may not be necessary for the SQL Cell use case but should be
 # considered for completeness.