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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Incompatibility with Python 3.15 alpha 7","articleBody":"We are already building RPM packages with Python 3.15 alpha 7 in Fedora Linux and I've found some issues and solutions for them.\n\n### call_count in Mock\n\nSince https://github.com/python/cpython/commit/850f95f, `call_count` is now calculated property, so setting `call_count = 0` in tests does not work as before and does not actually reset the counter.\n\nMy solution for `test_norma_callback`:\n\n```diff\n--- a/lib/matplotlib/tests/test_colors.py\n+++ b/lib/matplotlib/tests/test_colors.py\n@@ -1590,7 +1590,9 @@ def test_norm_callback():\n     assert increment.call_count == 2\n \n     # We only want autoscale() calls to send out one update signal\n-    increment.call_count = 0\n+    # Python 3.15: Mock.call_count is now a computed property based on\n+    # len(call_args_list), so setting call_count=0 no longer works.\n+    increment.reset_mock()\n     norm.autoscale([0, 1, 2])\n     assert increment.call_count == 1\n```\n\n### tolerances\n\nI have to increase tolerances in `test_pcolornearestunits` and `test_rcupdate` – I don't know whether this is realted to Python 3.15 or not though.\n\n```diff\n--- a/lib/matplotlib/tests/test_axes.py\n+++ b/lib/matplotlib/tests/test_axes.py\n@@ -1668,7 +1668,7 @@ def test_pcolornearest(fig_test, fig_ref):\n     ax.pcolormesh(x2, y2, Z, shading='nearest')\n \n \n-@check_figures_equal(extensions=[\"png\"])\n+@check_figures_equal(extensions=[\"png\"], tol=5.0)\n def test_pcolornearestunits(fig_test, fig_ref):\n     ax = fig_test.subplots()\n     x = [datetime.datetime.fromtimestamp(x * 3600) for x in range(10)]\n```\nand\n```diff\n--- a/lib/matplotlib/tests/test_backend_pgf.py\n+++ b/lib/matplotlib/tests/test_backend_pgf.py\n@@ -131,7 +131,7 @@ def test_rcupdate():\n                 'pgf.preamble': ('\\\\usepackage[utf8x]{inputenc}'\n                                  '\\\\usepackage[T1]{fontenc}'\n                                  '\\\\usepackage{sfmath}')}]\n-    tol = [0, 13.2] if _old_gs_version else [0, 0]\n+    tol = [0, 13.2] if _old_gs_version else [0, 5.0]  # Increased tolerance for Python 3.15\n     for i, rc_set in enumerate(rc_sets):\n         with mpl.rc_context(rc_set):\n             for substring, pkg in [('sfmath', 'sfmath'), ('utf8x', 'ucs')]:\n```\n\n### Silenced deprecation warnings\n\nI have to silence `gi.PyGIDeprecationWarning: GLib.unix_signal_add_full is deprecated; use GLibUnix.signal_add_full instead` and `DeprecationWarning: This process (pid=13234) is multi-threaded, use of fork() may lead to deadlocks in the child.`. Only the second one is coming from Python itself but both might be important for new releases of matplotlib.\n\n```diff\n--- a/lib/matplotlib/tests/test_backend_gtk3.py\n+++ b/lib/matplotlib/tests/test_backend_gtk3.py\n@@ -6,6 +6,8 @@ from unittest import mock\n \n \n @pytest.mark.backend(\"gtk3agg\", skip_on_importerror=True)\n+@pytest.mark.filterwarnings(\"ignore:.*unix_signal_add_full.*:gi.PyGIDeprecationWarning\")\n+@pytest.mark.filterwarnings(\"ignore:.*unix_signal_add_full.*:DeprecationWarning\")\n def test_correct_key():\n     pytest.xfail(\"test_widget_send_event is not triggering key_press_event\")\n \n```\nand\n```diff\n--- a/lib/matplotlib/tests/test_font_manager.py\n+++ b/lib/matplotlib/tests/test_font_manager.py\n@@ -230,6 +230,9 @@ def _model_handler(_):\n \n @pytest.mark.skipif(not hasattr(os, \"register_at_fork\"),\n                     reason=\"Cannot register at_fork handlers\")\n+# Python 3.15+ raises DeprecationWarning for fork in multi-threaded process\n+@pytest.mark.filterwarnings(\"ignore:.*multi-threaded.*fork.*:DeprecationWarning\")\n+@pytest.mark.filterwarnings(\"ignore:.*multi-threaded.*fork.*:RuntimeWarning\")\n def test_fork():\n     _model_handler(0)  # Make sure the font cache is filled.\n     ctx = multiprocessing.get_context(\"fork\")\n```","author":{"url":"https://github.com/frenzymadness","@type":"Person","name":"frenzymadness"},"datePublished":"2026-03-31T12:56:27.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":11},"url":"https://github.com/31429/matplotlib/issues/31429"}