René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"[Bug]: Regression in 3.11.0 with LinearSegmentedColormap","articleBody":"### Bug summary\n\nMinimal test case using `LinearSegmentedColormap` extracted from https://github.com/pyani-plus/pyani-plus/blob/v1.0.1/pyani_plus/plot_run.py which worked on macOS and Linux with eg `matplotlib==3.10.8` but breaks on `matplotlib-3.11.0` with:\n\n`ValueError: the values passed in the (value, color) pairs must increase monotonically from 0 to 1.`\n\nThe release notes API changes do not mention this, https://matplotlib.org/stable/api/prev_api_changes/api_changes_3.11.0.html\n\n### Code for reproduction\n\n```Python\nfrom matplotlib import colormaps, __version__\nfrom matplotlib.colors import LinearSegmentedColormap\n\nprint(f\"matplotlib {__version__}\")\n\nGREY = (0.7, 0.7, 0.7)\nDULL_BLUE = (0.137, 0.412, 0.737)\nWHITE = (1.0, 1.0, 1.0)\nDULL_RED = (0.659, 0.216, 0.231)\n\ncolormaps.register(\n    LinearSegmentedColormap.from_list(\n        \"spbnd_BuRd\",  # species boundary - blue to red\n        (\n            (0.00, GREY),  # 0% grey\n            (0.80, GREY),  # 80% grey (not meaningful)\n            (0.80, DULL_BLUE),  # 80% blue\n            (0.95, WHITE),  # 95% white (species boundary)\n            (1.00, DULL_RED),  # 100% red\n        ),\n    )\n)\n\nprint(\"Done\")\n```\n\n### Actual outcome\n\nIt now fails:\n\n```\n❯ pip install matplotlib==3.11.0\nCollecting matplotlib==3.11.0\n...\nSuccessfully installed matplotlib-3.11.0\n\n❯ python test.py\nmatplotlib 3.11.0\nTraceback (most recent call last):\n  File \"/Users/peterjc/miniforge3/envs/pyani-plus_py312/lib/python3.12/site-packages/matplotlib/colors.py\", line 1218, in from_list\n    r, g, b, a = to_rgba_array(colors).T\n                 ^^^^^^^^^^^^^^^^^^^^^\n  File \"/Users/peterjc/miniforge3/envs/pyani-plus_py312/lib/python3.12/site-packages/matplotlib/colors.py\", line 532, in to_rgba_array\n    rgba = np.array([to_rgba(cc) for cc in c])\n                     ^^^^^^^^^^^\n  File \"/Users/peterjc/miniforge3/envs/pyani-plus_py312/lib/python3.12/site-packages/matplotlib/colors.py\", line 345, in to_rgba\n    rgba = _to_rgba_no_colorcycle(c, alpha)\n           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n  File \"/Users/peterjc/miniforge3/envs/pyani-plus_py312/lib/python3.12/site-packages/matplotlib/colors.py\", line 364, in _to_rgba_no_colorcycle\n    if alpha is not None and not 0 \u003c= alpha \u003c= 1:\n                                 ^^^^^^^^^^^^^^^\nTypeError: '\u003c=' not supported between instances of 'int' and 'tuple'\n\nDuring handling of the above exception, another exception occurred:\n\nTraceback (most recent call last):\n  File \"/private/tmp/test.py\", line 12, in \u003cmodule\u003e\n    LinearSegmentedColormap.from_list(\n  File \"/Users/peterjc/miniforge3/envs/pyani-plus_py312/lib/python3.12/site-packages/matplotlib/colors.py\", line 1229, in from_list\n    raise ValueError(\nValueError: the values passed in the (value, color) pairs must increase monotonically from 0 to 1.\n```\n\n### Expected outcome\n\nThis used to work, eg\n\n```\n❯ pip install matplotlib==3.10.8\nCollecting matplotlib==3.10.8\n...\nSuccessfully installed matplotlib-3.10.8\n\n❯ python test.py\nmatplotlib 3.10.8\nDone\n```\n\n### Additional information\n\n_No response_\n\n### Operating system\n\n_No response_\n\n### Matplotlib Version\n\n3.11.0\n\n### Matplotlib Backend\n\nmacosx (or Linux tested via CI)\n\n### Python version\n\n3.12.10 (and Python 3.11 and 3.13 via CI)\n\n### Jupyter version\n\n_No response_\n\n### Installation\n\npip","author":{"url":"https://github.com/peterjc","@type":"Person","name":"peterjc"},"datePublished":"2026-06-23T09:50:49.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":9},"url":"https://github.com/31939/matplotlib/issues/31939"}