fix: refactor test_anywidget imports to avoid NameError when dependencies missing

- Reverted changes to bigframes/display/anywidget.py to keep source code clean (no mock injection).
- Refactored tests/unit/display/test_anywidget.py to avoid top-level import of bigframes.display.anywidget.
- Imported TableWidget inside the test method, ensuring pytest.importorskip runs first and skips the test file if dependencies are missing, preventing the NameError crash.

Author: shuoweil

bigframes/display/anywidget.py

@@ -43,10 +43,6 @@
     _ANYWIDGET_INSTALLED = True
 except Exception:
     _ANYWIDGET_INSTALLED = False
-    # Define dummy traitlets to avoid NameError at class definition time
-    import unittest.mock
-
-    traitlets = unittest.mock.MagicMock()
 
 _WIDGET_BASE: type[Any]
 if _ANYWIDGET_INSTALLED:

tests/unit/display/test_anywidget.py

@@ -18,13 +18,12 @@
 import pandas as pd
 import pytest
 
+import bigframes.dataframe
+
 # Skip if anywidget/traitlets not installed, though they should be in the dev env
 pytest.importorskip("anywidget")
 pytest.importorskip("traitlets")
 
-import bigframes.dataframe  # noqa: E402
-import bigframes.display.anywidget as anywidget  # noqa: E402
-
 
 class TestTableWidget:
     def test_navigation_to_invalid_page_resets_to_valid_page_without_deadlock(self):
@@ -34,6 +33,8 @@ def test_navigation_to_invalid_page_resets_to_valid_page_without_deadlock(self):
         This behavior relies on _set_table_html releasing the lock before updating self.page,
         preventing re-entrancy issues where the observer triggers a new update on the same thread.
         """
+        from bigframes.display import TableWidget
+
         mock_df = mock.create_autospec(bigframes.dataframe.DataFrame, instance=True)
         mock_df.columns = ["col1"]
         mock_df.dtypes = {"col1": "object"}
@@ -43,8 +44,8 @@ def test_navigation_to_invalid_page_resets_to_valid_page_without_deadlock(self):
         mock_df._block = mock_block
 
         # We mock _initial_load to avoid complex setup
-        with mock.patch.object(anywidget.TableWidget, "_initial_load"):
-            widget = anywidget.TableWidget(mock_df)
+        with mock.patch.object(TableWidget, "_initial_load"):
+            widget = TableWidget(mock_df)
 
         # Simulate "loaded data but unknown total rows" state
         widget.page_size = 10