René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"hexbin passes empty bins to reduce_C_function when mincnt is 0 or None","articleBody":"### Bug summary\n\nEmpty bins are passed to the C reducer when `mincnt` is zero or `None`. This causes reducers such as `np.sum` to produce spurious zero-valued bins, while custom reducers may raise an exception.\n\n### Code for reproduction\n\n```python\nimport numpy as np\nimport matplotlib.pyplot as plt\n\nx = [0.25]\ny = [0.25]\nc = [3.0]\nmincnt = 0\n\ndef reject_empty(values):\n    if len(values) == 0:\n        raise ValueError(\"empty bin passed to reducer\")\n    return float(np.sum(values))\n\nprint(\"Inputs:\", {\"x\": x, \"y\": y, \"C\": c, \"mincnt\": mincnt})\ntry:\n    fig, ax = plt.subplots()\n    collection = ax.hexbin(\n        x, y, C=c, gridsize=3, extent=(0, 1, 0, 1),\n        reduce_C_function=reject_empty, mincnt=mincnt\n    )\n    print(\"Bin offsets:\", collection.get_offsets().tolist())\n    print(\"Reduced values:\", collection.get_array().tolist())\n    plt.close(fig)\nexcept Exception as exc:\n    print(str(exc))\n```\n\n### Actual outcome\n\n```text\nInputs: {'x': [0.25], 'y': [0.25], 'C': [3.0], 'mincnt': 0}\nempty bin passed to reducer\n```\n\n### Expected outcome\n\nEmpty bins should not be passed to `reduce_C_function`. The output should contain only the non-empty bin:\n\n```text\nInputs: {'x': [0.25], 'y': [0.25], 'C': [3.0], 'mincnt': 0}\nBin offsets: [[0.333333333, 0.0]]\nReduced values: [3.0]\n```\n\n### Additional information\n\nThis is a regression introduced by PR #26113: https://github.com/matplotlib/matplotlib/pull/26113\n\nThe bin-inclusion condition was changed from `count \u003e mincnt` to `count \u003e= mincnt`. Consequently, zero-count bins are processed when `mincnt` is `0`. When `C` is provided and `mincnt` is `None`, it is internally set to `0`, producing the same behavior. Reducers such as `np.sum` return spurious zero-valued bins for empty inputs, while custom reducers may raise an exception.\n\n### Operating system\n\nLinux\n\n### Matplotlib Version\n\n3.11.1\n\n### Matplotlib Backend\n\nagg\n\n### Python version\n\n3.11.13\n\n### Jupyter version\n\nN/A\n\n### Installation\n\nN/A","author":{"url":"https://github.com/SuYanqi","@type":"Person","name":"SuYanqi"},"datePublished":"2026-07-20T10:07:58.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/32077/matplotlib/issues/32077"}