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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"[Bug]: Random timeout failures in CI","articleBody":"### Bug summary\n\nWe have been struggling with flaky timeout behavior for some time. See https://github.com/matplotlib/matplotlib/issues/29797. At some point we thought, we had fixed it though updating the runner, but it sometimes happens again. So it's not eventually solved. Possibly we have to revive https://github.com/matplotlib/matplotlib/pull/29981 and/or dig deeper.\n\nFailures look like\n```\n\u003e               raise TimeoutExpired(self.args, orig_timeout)\nE               subprocess.TimeoutExpired: Command '['C:\\\\hostedtoolcache\\\\windows\\\\Python\\\\3.12.10\\\\x64\\\\python.exe', '-c', \"import importlib.util;_spec = importlib.util.spec_from_file_location('matplotlib.tests.test_backends_interactive', 'D:\\\\\\\\a\\\\\\\\1\\\\\\\\s\\\\\\\\lib\\\\\\\\matplotlib\\\\\\\\tests\\\\\\\\test_backends_interactive.py');_module = importlib.util.module_from_spec(_spec);_spec.loader.exec_module(_module);_module._impl_test_lazy_auto_backend_selection()\"]' timed out after 20 seconds\n```\n\n\n### Code for reproduction\n\n```Python\n-\n```\n\n\nFor a start, let's track in which pipelines and for which tests the timeout can be observed:\n\n- Pytest Windows_py311 - test_lazy_auto_backend_selection [ref](https://dev.azure.com/matplotlib/matplotlib/_build/results?buildId=46205\u0026view=logs\u0026j=f92c32cd-721a-536e-3a87-a0c010db3f7f\u0026t=d4eecd58-7131-52e4-f793-e9e3ca32dedb)\n- Pytest Windows_py313 - test_interactive_backend[toolbar2-MPLBACKEND=tkagg-BACKEND_DEPS=tkinter] [ref](https://dev.azure.com/matplotlib/matplotlib/_build/results?buildId=46205\u0026view=logs\u0026j=0e51c8a9-4119-5244-b946-ee585b1b50f1\u0026t=e90dd38b-859f-5d2f-90bb-85e3d7473c7a)\n- Pytest Windows_py313 - test_fontcache_thread_safe [ref](https://dev.azure.com/matplotlib/matplotlib/_build/results?buildId=46205\u0026view=logs\u0026j=0e51c8a9-4119-5244-b946-ee585b1b50f1\u0026t=e90dd38b-859f-5d2f-90bb-85e3d7473c7a)\n- Pytest Windows_py313 - test_fallback_to_different_backend [ref](https://dev.azure.com/matplotlib/matplotlib/_build/results?buildId=46186\u0026j=0e51c8a9-4119-5244-b946-ee585b1b50f1\u0026t=e90dd38b-859f-5d2f-90bb-85e3d7473c7a)\n- Pytest Windows_py313 - test_blitting_events[MPLBACKEND=tkagg-BACKEND_DEPS=tkinter] [ref](https://dev.azure.com/matplotlib/matplotlib/_build/results?buildId=46186\u0026j=0e51c8a9-4119-5244-b946-ee585b1b50f1\u0026t=e90dd38b-859f-5d2f-90bb-85e3d7473c7a)\n- Pytest Windows_py313 test_blit [ref](https://dev.azure.com/matplotlib/matplotlib/_build/results?buildId=46184\u0026view=logs\u0026j=0e51c8a9-4119-5244-b946-ee585b1b50f1\u0026t=e90dd38b-859f-5d2f-90bb-85e3d7473c7a)\n- Pytest Windows_py313 test_fallback_to_different_backend [ref](https://dev.azure.com/matplotlib/matplotlib/_build/results?buildId=46184\u0026view=logs\u0026j=0e51c8a9-4119-5244-b946-ee585b1b50f1\u0026t=e90dd38b-859f-5d2f-90bb-85e3d7473c7a)\n- Pytest Windows_py313 test_backend_getattr[matplotlib.backends.backend_agg] [ref](https://dev.azure.com/matplotlib/matplotlib/_build/results?buildId=46184\u0026view=logs\u0026j=0e51c8a9-4119-5244-b946-ee585b1b50f1\u0026t=e90dd38b-859f-5d2f-90bb-85e3d7473c7a)\n- Pytest Windows_py312 test_interactive_thread_safety[MPLBACKEND=tkagg-BACKEND_DEPS=tkinter] [ref](https://dev.azure.com/matplotlib/matplotlib/_build/results?buildId=46183\u0026view=logs\u0026j=f92c32cd-721a-536e-3a87-a0c010db3f7f\u0026t=d4eecd58-7131-52e4-f793-e9e3ca32dedb)\n- Pytest Windows_py313 test_interactive_timers[MPLBACKEND=tkagg-BACKEND_DEPS=tkinter] [ref](https://dev.azure.com/matplotlib/matplotlib/_build/results?buildId=46171\u0026view=logs\u0026j=0e51c8a9-4119-5244-b946-ee585b1b50f1\u0026t=e90dd38b-859f-5d2f-90bb-85e3d7473c7aj)\n- Pytest Windows_py312 test_fontcache_thread_safe [ref](https://dev.azure.com/matplotlib/matplotlib/_build/results?buildId=46169\u0026view=logs\u0026j=f92c32cd-721a-536e-3a87-a0c010db3f7f\u0026t=d4eecd58-7131-52e4-f793-e9e3ca32dedb)\n- Pytest Windows_py313 test_blit [ref](https://dev.azure.com/matplotlib/matplotlib/_build/results?buildId=46169\u0026view=logs\u0026j=0e51c8a9-4119-5244-b946-ee585b1b50f1\u0026t=e90dd38b-859f-5d2f-90bb-85e3d7473c7a)\n- Pytest Windows_py312 test_qt_missing [ref](https://dev.azure.com/matplotlib/matplotlib/_build/results?buildId=46135\u0026view=logs\u0026j=f92c32cd-721a-536e-3a87-a0c010db3f7f\u0026t=d4eecd58-7131-52e4-f793-e9e3ca32dedb)\n\n### Observations\n- seems only to occur on windows, but with all python versions (3.11-3.13)\n- if it fails it's typically one test, but up to 4 timed out tests have been observed in a single pytest run\n- the Majority of PRs have at least one timeout\n- it happens in different tests every time but the common part is that they create subprocesses ","author":{"url":"https://github.com/timhoffm","@type":"Person","name":"timhoffm"},"datePublished":"2025-12-13T09:31:53.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":38},"url":"https://github.com/30851/matplotlib/issues/30851"}