Init FigureCanvasAgg._lastKey at class-level.

This avoids the need for getattr checks.  We explicitly don't do the
same for FigureCanvasAgg.renderer because 1) if _lastKey has been set to
a non-None value, then renderer has also been set and 2) initing
renderer to e.g. None would change the possible types of a public
attribute.

Author: anntzer

lib/matplotlib/backends/backend_agg.py

@@ -387,6 +387,8 @@ def post_processing(image, dpi):
 class FigureCanvasAgg(FigureCanvasBase):
     # docstring inherited
 
+    _lastKey = None  # Overwritten per-instance on the first draw.
+
     def copy_from_bbox(self, bbox):
         renderer = self.get_renderer()
         return renderer.copy_from_bbox(bbox)
@@ -412,8 +414,7 @@ def draw(self):
     def get_renderer(self, cleared=False):
         w, h = self.figure.bbox.size
         key = w, h, self.figure.dpi
-        reuse_renderer = (hasattr(self, "renderer")
-                          and getattr(self, "_lastKey", None) == key)
+        reuse_renderer = (self._lastKey == key)
         if not reuse_renderer:
             self.renderer = RendererAgg(w, h, self.figure.dpi)
             self._lastKey = key