René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"[Bug]: numerical instability of Path.arc for 0 degree or 360 degree arc","articleBody":"### Bug summary\n\nTrying to create an arc path of 0/360 degrees sometimes results in one of 360/0 degrees due to floating point weirdness.\n\n### Code for reproduction\n\n```Python\nfrom matplotlib import patches, path, pyplot\n\nfig, ax = pyplot.subplots(ncols=3)\nax[0].add_patch(patches.PathPatch(path.Path.arc(theta1=-90 - 1e-14, theta2=270)))\nax[0].set_title(\"arc(-90-1e-14, 270), should be a circle\")\nax[1].add_patch(patches.PathPatch(path.Path.arc(theta1=-90, theta2=270)))\nax[1].set_title(\"arc(-90, 270), is a circle\")\nax[2].add_patch(patches.PathPatch(path.Path.arc(theta1=-90, theta2=-90 - 1e-14)))\nax[2].set_title(\"arc(-90, -90-1e-14), should not be a circle\")\nfor a in ax:\n    a.set_xlim(-1, 1)\n    a.set_ylim(-1, 1)\n    a.set_aspect(\"equal\")\npyplot.show()\n```\n\n### Actual outcome\n\n![Image](https://github.com/user-attachments/assets/2108d052-c1fe-4df1-a54e-ffe5d59d1cc5)\n\n### Expected outcome\n\nI expect the first plot to be a circle, identical to the second.\nI expect the third plot to be empty\n\n### Additional information\n\nThe location of the bug is not hard to find:\nhttps://github.com/matplotlib/matplotlib/blob/6fc8169112a42e3f6f55793a6b56159cd8ff69d7/lib/matplotlib/path.py#L951-L956\n\nAt runtime the first example becomes `(-90 \u003c= -90 - 1e-14) == False`, which therefore skips the `+360` bump.\n\nWhat is much harder is to understand is what the intention is here.\nIf you can help me to understand what behaviour you'd like, I'd be happy to put together a PR for it.\n\nI think `theta2 \u003c theta1` always gives an unexpected result:\n- The docstring does not clarify what should happen, therefore my assumption is that it should return an anti-clockwise arc\n- The `np.floor` will mean it becomes the angle range [theta1, theta2+360], which gives the third plot which I believe should not be a circle\n- The special casing only treats `eta2 += 360`, not `eta2 -= 360`\n\nIt would be great if either the docstring could be clarified or a `ValueError` raised on unexpected inputs here.\n\n### Operating system\n\n_No response_\n\n### Matplotlib Version\n\n3.10.0\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\npip","author":{"url":"https://github.com/robtovey","@type":"Person","name":"robtovey"},"datePublished":"2025-04-21T05:13:04.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":13},"url":"https://github.com/29953/matplotlib/issues/29953"}