matplotlib · jkseppan · Jul 5, 2025
diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
@@ -337,7 +337,7 @@ jobs:
           if [[ "${{ matrix.python-version }}" == '3.13t' ]]; then
             export PYTHON_GIL=0
           fi
-          pytest -rfEsXR -n auto \
+          pytest -rfEsXR -n auto --dist=loadgroup \
             --maxfail=50 --timeout=300 --durations=25 \
             --cov-report=xml --cov=lib --log-level=DEBUG --color=yes
 

diff --git a/azure-pipelines.yml b/azure-pipelines.yml
@@ -119,8 +119,7 @@ stages:
               VS_VER=2022
 
               echo "##vso[task.setvariable variable=VS_COVERAGE_TOOL]$TOOL"
-
-              PYTHONFAULTHANDLER=1 pytest -rfEsXR -n 2 \
+              PYTHONFAULTHANDLER=1 pytest -rfEsXR -n 2 --dist=loadgroup \
                   --maxfail=50 --timeout=300 --durations=25 \
                   --junitxml=junit/test-results.xml --cov-report=xml --cov=lib
 

diff --git a/lib/matplotlib/testing/_markers.py b/lib/matplotlib/testing/_markers.py
@@ -47,3 +47,4 @@ def _checkdep_usetex() -> bool:
 needs_usetex = pytest.mark.skipif(
     not _checkdep_usetex(),
     reason="This test needs a TeX installation")
+starts_subprocess = pytest.mark.xdist_group("subprocess")
diff --git a/lib/matplotlib/tests/test_backend_inline.py b/lib/matplotlib/tests/test_backend_inline.py
@@ -5,11 +5,13 @@
 import pytest
 
 from matplotlib.testing import subprocess_run_for_testing
+from matplotlib.testing._markers import starts_subprocess
 
 nbformat = pytest.importorskip('nbformat')
 pytest.importorskip('nbconvert')
 pytest.importorskip('ipykernel')
 pytest.importorskip('matplotlib_inline')
+pytestmark = starts_subprocess
 
 
 def test_ipynb():

diff --git a/lib/matplotlib/tests/test_backend_nbagg.py b/lib/matplotlib/tests/test_backend_nbagg.py
@@ -3,12 +3,14 @@
 from tempfile import TemporaryDirectory
 
 import pytest
+from matplotlib.testing._markers import starts_subprocess
 
 from matplotlib.testing import subprocess_run_for_testing
 
 nbformat = pytest.importorskip('nbformat')
 pytest.importorskip('nbconvert')
 pytest.importorskip('ipykernel')
+pytestmark = starts_subprocess
 
 # From https://blog.thedataincubator.com/2016/06/testing-jupyter-notebooks/
 

diff --git a/lib/matplotlib/tests/test_backend_webagg.py b/lib/matplotlib/tests/test_backend_webagg.py
@@ -3,9 +3,11 @@
 import pytest
 
 import matplotlib.backends.backend_webagg_core
+from matplotlib.testing._markers import starts_subprocess
 from matplotlib.testing import subprocess_run_for_testing
 
 
+@starts_subprocess
 @pytest.mark.parametrize("backend", ["webagg", "nbagg"])
 def test_webagg_fallback(backend):
     pytest.importorskip("tornado")

diff --git a/lib/matplotlib/tests/test_backends_interactive.py b/lib/matplotlib/tests/test_backends_interactive.py
@@ -20,6 +20,10 @@
 from matplotlib import _c_internal_utils
 from matplotlib.backend_tools import ToolToggleBase
 from matplotlib.testing import subprocess_run_helper as _run_helper, is_ci_environment
+from matplotlib.testing._markers import starts_subprocess
+
+
+pytestmark = starts_subprocess
 
 
 class _WaitForStringPopen(subprocess.Popen):

diff --git a/lib/matplotlib/tests/test_basic.py b/lib/matplotlib/tests/test_basic.py
@@ -3,6 +3,7 @@
 import sys
 import textwrap
 
+from matplotlib.testing._markers import starts_subprocess
 from matplotlib.testing import subprocess_run_for_testing
 
 
@@ -28,6 +29,7 @@ def test_override_builtins():
     assert overridden <= ok_to_override
 
 
+@starts_subprocess
 def test_lazy_imports():
     source = textwrap.dedent("""
     import sys

diff --git a/lib/matplotlib/tests/test_determinism.py b/lib/matplotlib/tests/test_determinism.py
@@ -16,11 +16,15 @@
 from matplotlib.patches import Circle, PathPatch
 from matplotlib.path import Path
 from matplotlib.testing import subprocess_run_for_testing
-from matplotlib.testing._markers import needs_ghostscript, needs_usetex
+from matplotlib.testing._markers import (
+    needs_ghostscript, needs_usetex, starts_subprocess
+)
 import matplotlib.testing.compare
 from matplotlib.text import TextPath
 from matplotlib.transforms import IdentityTransform
 
+pytestmark = starts_subprocess
+
 
 def _save_figure(objects='mhip', fmt="pdf", usetex=False):
     mpl.use(fmt)

diff --git a/lib/matplotlib/tests/test_font_manager.py b/lib/matplotlib/tests/test_font_manager.py
@@ -18,6 +18,7 @@
     MSUserFontDirectories, _get_fontconfig_fonts, ttfFontProperty)
 from matplotlib import cbook, ft2font, pyplot as plt, rc_context, figure as mfigure
 from matplotlib.testing import subprocess_run_helper, subprocess_run_for_testing
+from matplotlib.testing._markers import starts_subprocess
 
 
 has_fclist = shutil.which('fc-list') is not None
@@ -288,6 +289,7 @@ def test_fontcache_thread_safe():
     subprocess_run_helper(_test_threading, timeout=10)
 
 
+@starts_subprocess
 def test_lockfilefailure(tmp_path):
     # The logic here:
     # 1. get a temp directory from pytest

diff --git a/lib/matplotlib/tests/test_matplotlib.py b/lib/matplotlib/tests/test_matplotlib.py
@@ -7,6 +7,7 @@
 
 import matplotlib
 from matplotlib.testing import subprocess_run_for_testing
+from matplotlib.testing._markers import starts_subprocess
 
 
 @pytest.mark.parametrize('version_str, version_tuple', [
@@ -19,6 +20,7 @@ def test_parse_to_version_info(version_str, version_tuple):
     assert matplotlib._parse_to_version_info(version_str) == version_tuple
 
 
+@starts_subprocess
 @pytest.mark.skipif(sys.platform == "win32",
                     reason="chmod() doesn't work as is on Windows")
 @pytest.mark.skipif(sys.platform != "win32" and os.geteuid() == 0,
@@ -37,6 +39,7 @@ def test_tmpconfigdir_warning(tmp_path):
         os.chmod(tmp_path, mode)
 
 
+@starts_subprocess
 def test_importable_with_no_home(tmp_path):
     subprocess_run_for_testing(
         [sys.executable, "-c",

diff --git a/lib/matplotlib/tests/test_preprocess_data.py b/lib/matplotlib/tests/test_preprocess_data.py
@@ -8,6 +8,8 @@
 from matplotlib.axes import Axes
 from matplotlib.testing import subprocess_run_for_testing
 from matplotlib.testing.decorators import check_figures_equal
+from matplotlib.testing._markers import starts_subprocess
+
 
 # Notes on testing the plotting functions itself
 # *   the individual decorated plotting functions are tested in 'test_axes.py'
@@ -245,6 +247,7 @@ def funcy(ax, x, y, z, t=None):
                          funcy.__doc__)
 
 
+@starts_subprocess
 def test_data_parameter_replacement():
     """
     Test that the docstring contains the correct *data* parameter stub

diff --git a/lib/matplotlib/tests/test_pyplot.py b/lib/matplotlib/tests/test_pyplot.py
@@ -11,7 +11,10 @@
 from matplotlib.testing import subprocess_run_for_testing
 from matplotlib import pyplot as plt
 
+from matplotlib.testing._markers import starts_subprocess
 
+
+@starts_subprocess
 def test_pyplot_up_to_date(tmp_path):
     pytest.importorskip("black")
 

diff --git a/lib/matplotlib/tests/test_rcparams.py b/lib/matplotlib/tests/test_rcparams.py
@@ -31,6 +31,7 @@
     _validate_linestyle,
     _listify_validator)
 from matplotlib.testing import subprocess_run_for_testing
+from matplotlib.testing._markers import starts_subprocess
 
 
 def test_rcparams(tmp_path):
@@ -528,6 +529,7 @@ def test_rcparams_reset_after_fail():
         assert mpl.rcParams['text.usetex'] is False
 
 
+@starts_subprocess
 @pytest.mark.skipif(sys.platform != "linux", reason="Linux only")
 def test_backend_fallback_headless_invalid_backend(tmp_path):
     env = {**os.environ,
@@ -545,6 +547,7 @@ def test_backend_fallback_headless_invalid_backend(tmp_path):
             env=env, check=True, stderr=subprocess.DEVNULL)
 
 
+@starts_subprocess
 @pytest.mark.skipif(sys.platform != "linux", reason="Linux only")
 def test_backend_fallback_headless_auto_backend(tmp_path):
     # specify a headless mpl environment, but request a graphical (tk) backend
@@ -567,6 +570,7 @@ def test_backend_fallback_headless_auto_backend(tmp_path):
     assert backend.strip().lower() == "agg"
 
 
+@starts_subprocess
 @pytest.mark.skipif(
     sys.platform == "linux" and not _c_internal_utils.xdisplay_is_valid(),
     reason="headless")

diff --git a/lib/matplotlib/tests/test_sphinxext.py b/lib/matplotlib/tests/test_sphinxext.py
@@ -7,6 +7,8 @@
 import sys
 
 from matplotlib.testing import subprocess_run_for_testing
+from matplotlib.testing._markers import starts_subprocess
+
 import pytest
 
 
@@ -16,6 +18,7 @@
 tinypages = Path(__file__).parent / 'data/tinypages'
 
 
+@starts_subprocess
 def build_sphinx_html(source_dir, doctree_dir, html_dir, extra_args=None):
     # Build the pages with warnings turned into errors
     extra_args = [] if extra_args is None else extra_args

diff --git a/lib/matplotlib/tests/test_texmanager.py b/lib/matplotlib/tests/test_texmanager.py
@@ -9,6 +9,7 @@
 from matplotlib.testing import subprocess_run_for_testing
 from matplotlib.testing._markers import needs_usetex
 from matplotlib.texmanager import TexManager
+from matplotlib.testing._markers import starts_subprocess
 
 
 def test_fontconfig_preamble():
@@ -63,6 +64,7 @@ def test_unicode_characters():
         fig.canvas.draw()
 
 
+@starts_subprocess
 @needs_usetex
 def test_openin_any_paranoid():
     completed = subprocess_run_for_testing(