Prevent blitting errors after canvas swap in RadioButtons and CheckButtons

This guards blitting-related functionality of RadioButtons and
CheckButtons behind a `self.canvas.supports_blit` check, so that we
don't get errors when trying to call blitting functionality on canvases
that don't support it.

Note: RadioButtons and CheckButtons do still not fully support canvas
swapping as they carry persitent state that depends on blitting
capability ("animated"). This is currently not updated when canvases are
 changed. However, this should at most lead to incorrect rendering
 in some edge cases after
  canvas swap. It will not error out anymore.

 In other words the "# TODO: make dynamic" is partially solved, and
 reduced to a smaller problem outlined in the new TODO.

Author: timhoffm

lib/matplotlib/widgets.py

@@ -1108,7 +1108,7 @@ def __init__(self, ax, labels, actives=None, *, useblit=True,
         if actives is None:
             actives = [False] * len(labels)
 
-        self._useblit = useblit and self.canvas.supports_blit  # TODO: make dynamic
+        self._useblit = useblit
 
         ys = np.linspace(1, 0, len(labels)+2)[1:-1]
 
@@ -1136,7 +1136,10 @@ def __init__(self, ax, labels, actives=None, *, useblit=True,
             **cbook.normalize_kwargs(check_props, collections.PathCollection),
             'marker': 'x',
             'transform': ax.transAxes,
-            'animated': self._useblit,
+            'animated': self._useblit and self.canvas.supports_blit,
+            # TODO: This may need an update when switching out the canvas.
+            #       Can set this to `_useblit` only and live with the animated=True
+            #       overhead on unsupported backends.
         }
         check_props.setdefault('facecolor', check_props.pop('color', 'black'))
         self._checks = ax.scatter([0.15] * len(ys), ys, **check_props)
@@ -1155,7 +1158,8 @@ def _clear(self, event):
         """Internal event handler to clear the buttons."""
         if self.ignore(event) or self.canvas.is_saving():
             return
-        self._save_blit_background(self.canvas.copy_from_bbox(self.ax.bbox))
+        if self._useblit and self.canvas.supports_blit:
+            self._save_blit_background(self.canvas.copy_from_bbox(self.ax.bbox))
         self.ax.draw_artist(self._checks)
 
     @_call_with_reparented_event
@@ -1260,7 +1264,7 @@ def set_active(self, index, state=None):
         self._checks.set_facecolor(facecolors)
 
         if self.drawon:
-            if self._useblit:
+            if self._useblit and self.canvas.supports_blit:
                 background = self._load_blit_background()
                 if background is not None:
                     self.canvas.restore_region(background)
@@ -1701,7 +1705,7 @@ def __init__(self, ax, labels, active=0, activecolor=None, *,
 
         ys = np.linspace(1, 0, len(labels) + 2)[1:-1]
 
-        self._useblit = useblit and self.canvas.supports_blit  # TODO: make dynamic
+        self._useblit = useblit
 
         label_props = _expand_text_props(label_props)
         self.labels = [
@@ -1716,7 +1720,11 @@ def __init__(self, ax, labels, active=0, activecolor=None, *,
             **radio_props,
             'marker': 'o',
             'transform': ax.transAxes,
-            'animated': self._useblit,
+            'animated': self._useblit and self.canvas.supports_blit,
+            # TODO: This may need an update when switching out the canvas.
+            #       Can set this to `_useblit` only and live with the animated=True
+            #       overhead on unsupported backends.
+
         }
         radio_props.setdefault('edgecolor', radio_props.get('color', 'black'))
         radio_props.setdefault('facecolor',
@@ -1743,7 +1751,8 @@ def _clear(self, event):
         """Internal event handler to clear the buttons."""
         if self.ignore(event) or self.canvas.is_saving():
             return
-        self._save_blit_background(self.canvas.copy_from_bbox(self.ax.bbox))
+        if self._useblit and self.canvas.supports_blit:
+            self._save_blit_background(self.canvas.copy_from_bbox(self.ax.bbox))
         self.ax.draw_artist(self._buttons)
 
     @_call_with_reparented_event
@@ -1836,7 +1845,7 @@ def set_active(self, index):
         self._buttons.set_facecolor(button_facecolors)
 
         if self.drawon:
-            if self._useblit:
+            if self._useblit and self.canvas.supports_blit:
                 background = self._load_blit_background()
                 if background is not None:
                     self.canvas.restore_region(background)