MNT: put changed canvas logic in private method with docstring

Author: tacaswell

lib/matplotlib/widgets.py

@@ -90,6 +90,22 @@ def ignore(self, event):
         """
         return not self.active
 
+    def _changed_canvas(self):
+        """
+        Someone has switched the canvas on us!
+
+        This happens if `savefig` needs to save to a format the previous
+        backend did not support (e.g. saving a figure using an Agg based
+        backend saved to a vector format).
+
+        Returns
+        -------
+        bool
+           True if the canvas has been changed.
+
+        """
+        return self.canvas is not self.ax.figure.canvas
+
 
 class AxesWidget(Widget):
     """
@@ -1088,7 +1104,7 @@ def __init__(self, ax, labels, actives=None, *, useblit=True,
 
     def _clear(self, event):
         """Internal event handler to clear the buttons."""
-        if self.ignore(event) or self.canvas is not self.ax.figure.canvas:
+        if self.ignore(event) or self._changed_canvas():
             return
         self._background = self.canvas.copy_from_bbox(self.ax.bbox)
         self.ax.draw_artist(self._checks)
@@ -1700,7 +1716,7 @@ def __init__(self, ax, labels, active=0, activecolor=None, *,
 
     def _clear(self, event):
         """Internal event handler to clear the buttons."""
-        if self.ignore(event) or self.canvas is not self.ax.figure.canvas:
+        if self.ignore(event) or self._changed_canvas():
             return
         self._background = self.canvas.copy_from_bbox(self.ax.bbox)
         self.ax.draw_artist(self._buttons)
@@ -1971,7 +1987,7 @@ def __init__(self, ax, horizOn=True, vertOn=True, useblit=False,
 
     def clear(self, event):
         """Internal event handler to clear the cursor."""
-        if self.ignore(event) or self.canvas is not self.ax.figure.canvas:
+        if self.ignore(event) or self._changed_canvas():
             return
         if self.useblit:
             self.background = self.canvas.copy_from_bbox(self.ax.bbox)