TST : ensure canvas drawn before testing widets

Widgets assume that the canvas has been drawn atleast once
before they are called (which is reasonable for interactive usage
as the user needs to be able to _see_ the gui before they can
interact with it).  Ensure that the tests draw the canvas
before trying to use the widgets.  Doing this allows for removing
some of the error handling logic from the widgets.

lib/matplotlib/tests/test_widgets.py

@@ -65,8 +65,9 @@ def get_event(ax, button=1, xdata=0, ydata=0, key=None, step=1):
 
 @cleanup
 def check_rectangle(**kwargs):
-    ax = plt.gca()
+    fig, ax = plt.subplots(1, 1)
     ax.plot([0, 200], [0, 200])
+    ax.figure.canvas.draw()
 
     def onselect(epress, erelease):
         ax._got_onselect = True
@@ -100,8 +101,9 @@ def test_rectangle_selector():
 
 @cleanup
 def check_span(*args, **kwargs):
-    ax = plt.gca()
+    fig, ax = plt.subplots(1, 1)
     ax.plot([0, 200], [0, 200])
+    ax.figure.canvas.draw()
 
     def onselect(vmin, vmax):
         ax._got_onselect = True
@@ -140,8 +142,10 @@ def test_span_selector():
 
 @cleanup
 def check_lasso_selector(**kwargs):
+    fig, ax = plt.subplots(1, 1)
     ax = plt.gca()
     ax.plot([0, 200], [0, 200])
+    ax.figure.canvas.draw()
 
     def onselect(verts):
         ax._got_onselect = True

lib/matplotlib/widgets.py

@@ -1182,16 +1182,10 @@ def update(self):
             if self.background is not None:
                 self.canvas.restore_region(self.background)
             for artist in self.artists:
-                try:
-                    self.ax.draw_artist(artist)
-                except AssertionError:
-                    self.canvas.draw_idle()
-                    return False
-            try:
-                self.canvas.blit(self.ax.bbox)
-            except AttributeError:
-                self.canvas.draw_idle()
-                return False
+                self.ax.draw_artist(artist)
+
+            self.canvas.blit(self.ax.bbox)
+
         else:
             self.canvas.draw_idle()
         return False