FIX: don't fail on first show if animation already exhausted

tacaswell · tacaswell · commit 5471db1f4af6 · 2021-08-06T12:27:14.000-04:00
If we need to get the first frame in `_init_draw()` (because the user did not pass an initialization function) fails, warn instead of raising. closes #17770
diff --git a/lib/matplotlib/animation.py b/lib/matplotlib/animation.py
@@ -1666,8 +1666,21 @@ def _init_draw(self):
         # For blitting, the init_func should return a sequence of modified
         # artists.
         if self._init_func is None:
-            self._draw_frame(next(self.new_frame_seq()))
-
+            try:
+                frame_data = next(self.new_frame_seq())
+            except StopIteration:
+                # we can't start the iteration, it may have already been
+                # exhausted by a previous save or just be 0 length.
+                # warn and bail.
+                warnings.warn(
+                    "Can not start iterating the frames for the initial draw. "
+                    "This can be caused by passing in a 0 length sequence "
+                    "for *frames*.\n\n"
+                    "If you passed *frames* as a generator "
+                    "it may be exhausted due to a previous display or save."
+                )
+                return
+            self._draw_frame(frame_data)
         else:
             self._drawn_artists = self._init_func()
             if self._blit:
diff --git a/lib/matplotlib/tests/test_animation.py b/lib/matplotlib/tests/test_animation.py
@@ -359,3 +359,36 @@ def animate(i):
 
     with pytest.raises(RuntimeError):
         animation.FuncAnimation(fig, animate, blit=True)
+
+
+def test_exhausted_animation(tmpdir):
+    fig, ax = plt.subplots()
+
+    def update(frame):
+        return []
+
+    anim = animation.FuncAnimation(
+        fig, update, frames=iter(range(10)), repeat=False,
+        cache_frame_data=False
+    )
+
+    with tmpdir.as_cwd():
+        anim.save("test.gif", writer='pillow')
+
+    with pytest.warns(UserWarning, match="exhausted"):
+        anim._start()
+
+
+def test_no_frame_warning(tmpdir):
+    fig, ax = plt.subplots()
+
+    def update(frame):
+        return []
+
+    anim = animation.FuncAnimation(
+        fig, update, frames=[], repeat=False,
+        cache_frame_data=False
+    )
+
+    with pytest.warns(UserWarning, match="exhausted"):
+        anim._start()
