René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"CallbackRegistry should not silently drop duplicate callbacks","articleBody":"\u003c!--To help us understand and resolve your issue, please fill out the form to the best of your ability.--\u003e\r\n\u003c!--You can feel free to delete the sections that do not apply.--\u003e\r\n\r\n### Bug report\r\n\r\n**Bug summary**\r\n\r\nCurrently, if a same function is connected twice to the same signal and same callbackregisry, the second connection is silently dropped, per https://github.com/matplotlib/matplotlib/blob/b6a6414522f20a1f1b458ebe296b178856662dda/lib/matplotlib/cbook/__init__.py#L208-L210\r\nThis seems not so nice and a bit surprising (as in https://github.com/matplotlib/matplotlib/issues/15785#issuecomment-838512093); we should deprecate that behavior and either\r\n- (preferred) just always perform the connection, even if duplicated; it's the user's job to keep track of duplicates if they want; or\r\n- (less preferred) add something like `keep_duplicates={None,True,False}` where None (default) raises an exception for duplicates, True keeps them, False drops them.\r\n\r\n\u003c!--A short 1-2 sentences that succinctly describes the bug--\u003e\r\n\r\n**Code for reproduction**\r\n\r\n\u003c!--A minimum code snippet required to reproduce the bug.\r\nPlease make sure to minimize the number of dependencies required, and provide\r\nany necessary plotted data.\r\nAvoid using threads, as Matplotlib is (explicitly) not thread-safe.--\u003e\r\n\r\n```python\r\nfor _ in range(2): gcf().canvas.mpl_connect(\"button_press_event\", print)\r\nshow()\r\n```\r\nand click on the canvas.\r\n\r\n**Actual outcome**\r\n\r\nThe event is printed once.\r\n\r\n**Expected outcome**\r\n\r\nThe event is printed twice.\r\n\r\n**Matplotlib version**\r\n\u003c!--Please specify your platform and versions of the relevant libraries you are using:--\u003e\r\n  * Operating system: linux\r\n  * Matplotlib version (`import matplotlib; print(matplotlib.__version__)`): HEAD (3.4.x+)\r\n  * Matplotlib backend (`print(matplotlib.get_backend())`): any\r\n  * Python version: 39\r\n  * Jupyter version (if applicable): \r\n  * Other libraries: \r\n\r\n\u003c!--Please tell us how you installed matplotlib and python e.g., from source, pip, conda--\u003e\r\n\u003c!--If you installed from conda, please specify which channel you used if not the default--\u003e\r\n\r\n","author":{"url":"https://github.com/anntzer","@type":"Person","name":"anntzer"},"datePublished":"2021-05-12T11:57:12.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":5},"url":"https://github.com/20210/matplotlib/issues/20210"}