Warn if an animation is gc'd before doing anything.

QuLogic · QuLogic · commit 9cf0c09a732e · 2020-09-11T17:47:58.000-04:00
Fixes #18438.
diff --git a/lib/matplotlib/animation.py b/lib/matplotlib/animation.py
@@ -28,6 +28,7 @@
 import sys
 from tempfile import TemporaryDirectory
 import uuid
+import warnings
 
 import numpy as np
 
@@ -907,6 +908,8 @@ class Animation:
     """
 
     def __init__(self, fig, event_source=None, blit=False):
+        self._draw_was_started = False
+
         self._fig = fig
         # Disables blitting for backends that don't support it.  This
         # allows users to request it if available, but still have a
@@ -931,6 +934,14 @@ def __init__(self, fig, event_source=None, blit=False):
         if self._blit:
             self._setup_blit()
 
+    def __del__(self):
+        if not getattr(self, '_draw_was_started', True):
+            warnings.warn(
+                'Animation was deleted without rendering anything. This is '
+                'most likely unintended. To prevent deletion, assign the '
+                'Animation to a variable that exists for as long as you need '
+                'the Animation.')
+
     def _start(self, *args):
         """
         Starts interactive animation. Adds the draw frame command to the GUI
@@ -1166,7 +1177,7 @@ def _draw_next_frame(self, framedata, blit):
     def _init_draw(self):
         # Initial draw to clear the frame. Also used by the blitting code
         # when a clean base is required.
-        pass
+        self._draw_was_started = True
 
     def _pre_draw(self, framedata, blit):
         # Perform any cleaning or whatnot before the drawing of the frame.
@@ -1484,6 +1495,7 @@ def __init__(self, fig, artists, *args, **kwargs):
         super().__init__(fig, *args, **kwargs)
 
     def _init_draw(self):
+        super()._init_draw()
         # Make all the artists involved in *any* frame invisible
         figs = set()
         for f in self.new_frame_seq():
@@ -1695,6 +1707,7 @@ def gen():
                 return gen()
 
     def _init_draw(self):
+        super()._init_draw()
         # Initialize the drawing either using the given init_func or by
         # calling the draw function with the first item of the frame sequence.
         # For blitting, the init_func should return a sequence of modified
diff --git a/lib/matplotlib/tests/test_animation.py b/lib/matplotlib/tests/test_animation.py
@@ -1,3 +1,4 @@
+import gc
 import os
 from pathlib import Path
 import subprocess
@@ -75,6 +76,14 @@ def test_null_movie_writer():
     assert writer._count == num_frames
 
 
+def test_animation_delete():
+    anim = make_animation(frames=5)
+
+    with pytest.warns(Warning, match='Animation was deleted'):
+        del anim
+        gc.collect()
+
+
 def test_movie_writer_dpi_default():
     class DummyMovieWriter(animation.MovieWriter):
         def _run(self):
