MNT: be explicitly strict in FunctionAnimation

tacaswell · tacaswell · commit d42e2e85029c · 2016-04-25T23:46:44.000-04:00
If the user functions return None, raise RuntimeError. Suggested in #6332
diff --git a/lib/matplotlib/animation.py b/lib/matplotlib/animation.py
@@ -1113,8 +1113,8 @@ class FuncAnimation(TimedAnimation):
     results of drawing from the first item in the frames sequence will be
     used. This function will be called once before the first frame.
 
-    If blit=True, *func* and *init_func* should return an iterable of
-    drawables to clear.
+    If blit=True, *func* and *init_func* must return an iterable of
+    artists to be re-drawn.
 
     *kwargs* include *repeat*, *repeat_delay*, and *interval*:
     *interval* draws a new frame every *interval* milliseconds.
@@ -1195,6 +1195,9 @@ def _init_draw(self):
         else:
             self._drawn_artists = self._init_func()
             if self._blit:
+                if self._drawn_artists is None:
+                    raise RuntimeError('The init_func must return a '
+                                       'sequence of Artist objects.')
                 for a in self._drawn_artists:
                     a.set_animated(self._blit)
         self._save_seq = []
@@ -1210,6 +1213,9 @@ def _draw_frame(self, framedata):
         # Call the func with framedata and args. If blitting is desired,
         # func needs to return a sequence of any artists that were modified.
         self._drawn_artists = self._func(framedata, *self._args)
-        if self._blit:
+        if self._blit and self._drawn_artists:
+            if self._drawn_artists is None:
+                    raise RuntimeError('The animation function must return a '
+                                       'sequence of Artist objects.')
             for a in self._drawn_artists:
                 a.set_animated(self._blit)
