René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"`savefig` has sideeffects","articleBody":"In my investigation of image comparison fails with pytest and xdist plugin I have discovered strange behavior. By changing the extensions order of the image comparison tests you will receive different output (and of course test failures).\n\nI made extensions shuffle [like this](https://github.com/Kojoley/matplotlib/commit/6a361eb144003cadef1372584ccfa3b11183c072) and got [these results](https://travis-ci.org/Kojoley/matplotlib/jobs/148677312). From the log you can see that mostly affected are legend tests on svg backend. So any baseline image from the tests with `legend` call is spoiled by pdf backend, except the `legend` + `tightbox`.\n\nI tried to compare figure state before and after `savefig` call but run into issue #6870.\n\nHere is the way to reproduce the problem. You can compare output of the snippet with baseline images of `matplotlib.tests.test_legend.test_legend_auto1` test, if you want.\n\n``` python\n# python savefig_sideeffects.py svg svg pdf svg\n# diff -u 0.svg 1.svg\n# diff -u 1.svg 3.svg\n# SOURCE_DATE_EPOCH=0 python savefig_sideeffects.py pdf pdf svg pdf\n\nimport numpy as np\nimport matplotlib\nmatplotlib.use('agg')\nimport matplotlib.pylab as plt\n\n\n# matplotlib.testing.decorators.ImageComparisonTest.remove_text\ndef remove_text(figure):\n    from matplotlib import ticker\n    figure.suptitle(\"\")\n    for ax in figure.get_axes():\n        ax.set_title(\"\")\n        ax.xaxis.set_major_formatter(ticker.NullFormatter())\n        ax.xaxis.set_minor_formatter(ticker.NullFormatter())\n        ax.yaxis.set_major_formatter(ticker.NullFormatter())\n        ax.yaxis.set_minor_formatter(ticker.NullFormatter())\n        try:\n            ax.zaxis.set_major_formatter(ticker.NullFormatter())\n            ax.zaxis.set_minor_formatter(ticker.NullFormatter())\n        except AttributeError:\n            pass\n\n\n# matplotlib.tests.test_legend.test_legend_auto1\ndef test_legend_auto1():\n    'Test automatic legend placement'\n    fig = plt.figure()\n    ax = fig.add_subplot(111)\n    x = np.arange(100)\n    ax.plot(x, 50 - x, 'o', label='y=1')\n    ax.plot(x, x - 50, 'o', label='y=-1')\n    ax.legend(loc=0)\n    return fig\n\n\ndef prepare():\n    import matplotlib.testing\n    matplotlib._init_tests()\n\n    # matplotlib.testing.decorators.ImageComparisonTest.setup_class\n    matplotlib.testing.setup()\n    matplotlib.style.use('classic')\n    matplotlib.testing.set_font_settings_for_testing()\n\n    fig = test_legend_auto1()\n\n    remove_text(fig)\n\n    return fig\n\n\ndef test(exts):\n    fig = prepare()\n\n    # this will stick svg path ids with the same values\n    matplotlib.rc('svg', hashsalt='asdf')\n\n    for p in enumerate(exts):\n        fig.savefig('%d.%s' % p)\n\n\nif __name__ == '__main__':\n    import sys\n    test(sys.argv[1:])\n```\n\nThis is the demonstration of the issue, where `savefig` with pdf backend has sideeffect on the next `savefig` with svg backend:\n\n``` patch\n\u003epython savefig_sideeffects.py svg svg pdf svg\n\n\u003ediff -u 0.svg 1.svg\n\n\u003ediff -u 1.svg 3.svg\n--- 1.svg       2016-08-04 00:29:52.429050200 +0300\n+++ 3.svg       2016-08-04 00:29:52.537061000 +0300\n@@ -462,18 +462,18 @@\n    \u003c/g\u003e\n    \u003cg id=\"legend_1\"\u003e\n     \u003cg id=\"patch_7\"\u003e\n-     \u003cpath d=\"M 424.413 239.2965\n-L 511.2 239.2965\n-L 511.2 192.7035\n-L 424.413 192.7035\n+     \u003cpath d=\"M 424.469375 239.25225\n+L 511.2 239.25225\n+L 511.2 192.74775\n+L 424.469375 192.74775\n z\n \" style=\"fill:#ffffff;stroke:#000000;stroke-linejoin:miter;\"/\u003e\n     \u003c/g\u003e\n     \u003cg id=\"line2d_29\"/\u003e\n     \u003cg id=\"line2d_30\"\u003e\n      \u003cg\u003e\n-      \u003cuse style=\"fill:#0000ff;stroke:#000000;stroke-width:0.5;\" x=\"434.493\" xlink:href=\"#mb9df33688a\" y=\"204.36525\"/\u003e\n-      \u003cuse style=\"fill:#0000ff;stroke:#000000;stroke-width:0.5;\" x=\"454.653\" xlink:href=\"#mb9df33688a\" y=\"204.36525\"/\u003e\n+      \u003cuse style=\"fill:#0000ff;stroke:#000000;stroke-width:0.5;\" x=\"434.549375\" xlink:href=\"#mb9df33688a\" y=\"204.4095\"/\u003e\n+      \u003cuse style=\"fill:#0000ff;stroke:#000000;stroke-width:0.5;\" x=\"454.709375\" xlink:href=\"#mb9df33688a\" y=\"204.4095\"/\u003e\n      \u003c/g\u003e\n     \u003c/g\u003e\n     \u003cg id=\"text_1\"\u003e\n@@ -520,7 +520,7 @@\n z\n \" id=\"DejaVuSans-31\"/\u003e\n      \u003c/defs\u003e\n-     \u003cg transform=\"translate(470.493 209.40525)scale(0.144 -0.144)\"\u003e\n+     \u003cg transform=\"translate(470.549375 209.4495)scale(0.144 -0.144)\"\u003e\n       \u003cuse xlink:href=\"#DejaVuSans-79\"/\u003e\n       \u003cuse x=\"59.179688\" xlink:href=\"#DejaVuSans-3d\"/\u003e\n       \u003cuse x=\"142.96875\" xlink:href=\"#DejaVuSans-31\"/\u003e\n@@ -529,8 +529,8 @@\n     \u003cg id=\"line2d_31\"/\u003e\n     \u003cg id=\"line2d_32\"\u003e\n      \u003cg\u003e\n-      \u003cuse style=\"fill:#008000;stroke:#000000;stroke-width:0.5;\" x=\"434.493\" xlink:href=\"#m3d71661f95\" y=\"225.50175\"/\u003e\n-      \u003cuse style=\"fill:#008000;stroke:#000000;stroke-width:0.5;\" x=\"454.653\" xlink:href=\"#m3d71661f95\" y=\"225.50175\"/\u003e\n+      \u003cuse style=\"fill:#008000;stroke:#000000;stroke-width:0.5;\" x=\"434.549375\" xlink:href=\"#m3d71661f95\" y=\"225.546\"/\u003e\n+      \u003cuse style=\"fill:#008000;stroke:#000000;stroke-width:0.5;\" x=\"454.709375\" xlink:href=\"#m3d71661f95\" y=\"225.546\"/\u003e\n      \u003c/g\u003e\n     \u003c/g\u003e\n     \u003cg id=\"text_2\"\u003e\n@@ -543,7 +543,7 @@\n z\n \" id=\"DejaVuSans-2d\"/\u003e\n      \u003c/defs\u003e\n-     \u003cg transform=\"translate(470.493 230.54175)scale(0.144 -0.144)\"\u003e\n+     \u003cg transform=\"translate(470.549375 230.586)scale(0.144 -0.144)\"\u003e\n       \u003cuse xlink:href=\"#DejaVuSans-79\"/\u003e\n       \u003cuse x=\"59.179688\" xlink:href=\"#DejaVuSans-3d\"/\u003e\n       \u003cuse x=\"142.96875\" xlink:href=\"#DejaVuSans-2d\"/\u003e\n```\n\nSame happens to pdf after svg.\n\nBy accident, while fixing issue #6870, it seems that I have fixed this too. Test above now passes but there are 505 SVG baseline images affected and need update. I will open PR for this soon.\n","author":{"url":"https://github.com/Kojoley","@type":"Person","name":"Kojoley"},"datePublished":"2016-08-03T22:41:48.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":4},"url":"https://github.com/6899/matplotlib/issues/6899"}