René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"[Bug]: Event loop behavior differs across backends","articleBody":"### Bug summary\n\nI'm working on fixing several bugs with the macOS backend. I am trying to understand how `show()`, `start_event_loop()`, and `stop_event_loop()` should interact with each other.\n\nI had several questions that I couldn't find in the documentation:\n\n1) If `show(block=True)` is called, should `stop_event_loop()` make it return?\n2) If `start_event_loop()` is called on a figure, should closing all windows make it return?\n3) What should be the behavior if `start_event_loop()` or `show(block=True)` is called when an event loop is already running from `show()` or another `start_event_loop()`?\n4) If `stop_event_loop()` is called without an event loop running, it appears to be a no-op across all backends. Should it instead throw an exception?\n\nI wrote the attached code to test these. However, I am seeing different results from different backends. See charts in the \"Actual Results\" section.\n\n### Code for reproduction\n\n```Python\nimport matplotlib.pyplot as plt\nimport matplotlib\nimport sys\n\nexample_index = int(sys.argv[1])\n\nbackend, mode = (\n    ( \"QtAgg\",  \"show\" ),\n    ( \"TkAgg\",  \"show\" ),\n    ( \"macosx\", \"show\" ),\n    ( \"QtAgg\",  \"start_event_loop\" ),\n    ( \"TkAgg\",  \"start_event_loop\" ),\n    ( \"macosx\", \"start_event_loop\" ),\n)[ example_index ]\n\nprint(f\"backend = '{backend}', mode = '{mode}'\")\n\n\nmatplotlib.use(backend)\n\nfig, ax = plt.subplots()\nax.set_title(\"Click anywhere to continue\")\nax.plot([1, 2, 3])\n\nclicked = []\n\ndef on_click(event):\n    clicked.append((event.xdata, event.ydata))\n    print(\"on_click\")\n    fig.canvas.stop_event_loop()\n\nfig.canvas.mpl_connect('button_press_event', on_click)\n\nfig, ax = plt.subplots()\n\nif (mode == \"show\"):\n    plt.show()\nelse:\n    plt.show(block=False)\n    fig.canvas.start_event_loop()\n\nif (len(clicked) \u003e 0):\n    print(f\"User clicked at: {clicked[0]}\")\nelse:\n    print(f\"Event loop stopped without click\")\n```\n\n### Actual outcome\n\n#### Does closing both windows stop the event loop?\n\n| `backend` | `mode` | Result |\n|-|-|-|\n| QtAgg | show | Yes |\n| TkAgg | show | Yes |\n| macosx | show | Yes |\n| QtAgg | start_event_loop | No |\n| TkAgg | start_event_loop | **Yes** |\n| macosx | start_event_loop | No |\n \n\n#### Does clicking on Figure 1 stop the event loop?\n\n| `backend` | `mode` | Result |\n|-|-|-|\n| QtAgg | show | No |\n| TkAgg | show | **Yes** |\n| macosx | show | No |\n| QtAgg | start_event_loop | Yes |\n| TkAgg | start_event_loop | Yes |\n| macosx | start_event_loop | Yes |\n\n### Expected outcome\n\nAll backends should operate with the same behavior.\n\n### Additional information\n\n_No response_\n\n### Operating system\n\n_No response_\n\n### Matplotlib Version\n\nDev\n\n### Matplotlib Backend\n\n_No response_\n\n### Python version\n\n_No response_\n\n### Jupyter version\n\n_No response_\n\n### Installation\n\nNone","author":{"url":"https://github.com/iccir","@type":"Person","name":"iccir"},"datePublished":"2026-06-09T02:18:06.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/31858/matplotlib/issues/31858"}