FIX: Handle AxesWidget cleanup after failed init (matplotlib#30935)

* FIX: Handle AxesWidget cleanup after failed init

* Apply code review suggestions

* Update lib/matplotlib/tests/test_widgets.py

Co-authored-by: Tim Hoffmann <2836374+timhoffm@users.noreply.github.com>

* Remove __del__ method

* Remove AxesWidget.__del__ from widgets.pyi

---------

Co-authored-by: Tim Hoffmann <2836374+timhoffm@users.noreply.github.com>

Author: larrybradley

lib/matplotlib/tests/test_widgets.py

@@ -517,6 +517,17 @@ def test_rectangle_resize_square_center_aspect(ax, use_data_coordinates):
                                        46.25, 133.75])
 
 
+def test_axeswidget_del_on_failed_init():
+    """
+    Test that an unraisable exception is not created when initialization
+    fails.
+    """
+    # Pytest would fail the test if such an exception occurred.
+    fig, ax = plt.subplots()
+    with pytest.raises(TypeError, match="unexpected keyword argument 'undefined'"):
+        widgets.Button(ax, undefined='bar')
+
+
 def test_ellipse(ax):
     """For ellipse, test out the key modifiers"""
     tool = widgets.EllipseSelector(ax, grab_range=10, interactive=True)

lib/matplotlib/widgets.py

@@ -12,6 +12,7 @@
 import enum
 import functools
 import itertools
+import weakref
 from numbers import Integral, Number
 
 from cycler import cycler
@@ -118,10 +119,6 @@ def __init__(self, ax):
         self._cids = []
         self._blit_background_id = None
 
-    def __del__(self):
-        if self._blit_background_id is not None:
-            self.canvas._release_blit_background_id(self._blit_background_id)
-
     canvas = property(
         lambda self: getattr(self.ax.get_figure(root=True), 'canvas', None)
     )
@@ -162,7 +159,9 @@ def _save_blit_background(self, background):
         good enough for all existing widgets.
         """
         if self._blit_background_id is None:
-            self._blit_background_id = self.canvas._get_blit_background_id()
+            bbid = self.canvas._get_blit_background_id()
+            weakref.finalize(self, self.canvas._release_blit_background_id, bbid)
+            self._blit_background_id = bbid
         self.canvas._blit_backgrounds[self._blit_background_id] = background
 
     def _load_blit_background(self):

lib/matplotlib/widgets.pyi

@@ -35,7 +35,6 @@ class Widget:
 class AxesWidget(Widget):
     ax: Axes
     def __init__(self, ax: Axes) -> None: ...
-    def __del__(self) -> None: ...
     @property
     def canvas(self) -> FigureCanvasBase | None: ...
     def connect_event(self, event: Event, callback: Callable) -> None: ...