René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"[Bug]: Pickle round-tripping a figure incorrectly restores its original DPI instead of a DPI explicitly changed via Figure.set_dpi().","articleBody":"### Bug summary\n\nPickle round-tripping a figure incorrectly restores its original DPI instead of a DPI explicitly changed via `Figure.set_dpi()`.\n\n### Code for reproduction\n\n```python\nimport pickle\nimport matplotlib.pyplot as plt\n\nfig, ax = plt.subplots(figsize=(5, 3), dpi=100)\nax.plot([0, 1, 2, 3], [0, 1, 4, 9], label=\"quadratic\")\nax.set_title(\"DPI round trip\")\nax.legend()\n\nprint(\"Original DPI:\", fig.get_dpi())\nfig.set_dpi(150)\nprint(\"DPI after public set_dpi(150):\", fig.get_dpi())\n\nrestored = pickle.loads(pickle.dumps(fig))\nprint(\"DPI after pickle round trip:\", restored.get_dpi())\nprint(\"Line count:\", len(restored.axes[0].lines))\n\nplt.close(fig)\nplt.close(restored)\n```\n\n### Actual outcome\n\n```text\nOriginal DPI: 100\nDPI after public set_dpi(150): 150\nDPI after pickle round trip: 100\nLine count: 1\n```\n\n### Expected outcome\n\nThe explicitly configured DPI should be preserved across pickle round-tripping:\n\n```text\nOriginal DPI: 100\nDPI after public set_dpi(150): 150\nDPI after pickle round trip: 150\nLine count: 1\n```\n\n### Additional information\n\nThis regression was introduced by [PR #23476](https://github.com/matplotlib/matplotlib/pull/23476).\n\nThe new code replaces the current DPI with `_original_dpi` whenever that state entry exists. This correctly discards a transient DPI change made for a canvas device-pixel ratio, but `_original_dpi` is established independently of later public calls to `Figure.set_dpi()`. Consequently, a figure intentionally changed from 100 to 150 DPI is serialized with 100 DPI instead of 150 DPI, causing a loss of public figure state.\n\n### Operating system\n\nLinux\n\n### Matplotlib Version\n\n3.11.0\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/RippleTester","@type":"Person","name":"RippleTester"},"datePublished":"2026-07-17T10:07:26.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":3},"url":"https://github.com/32053/matplotlib/issues/32053"}