matplotlib · jkseppan · Jul 5, 2025 · Jul 9, 2025 · Jul 9, 2025 · Jul 9, 2025
diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
@@ -40,54 +40,16 @@ jobs:
       )
     permissions:
       contents: read
-    name: "Python ${{ matrix.python-version }} on ${{ matrix.os }} ${{ matrix.name-suffix }}"
+    name: "Python ${{ matrix.python-version }} on ${{ matrix.os }}, ${{ matrix.test_type}} ${{ matrix.name-suffix }}"
     runs-on: ${{ matrix.os }}
 
     strategy:
       fail-fast: false
       matrix:
         include:
-          - name-suffix: "(Minimum Versions)"
-            os: ubuntu-22.04
-            python-version: '3.11'
-            extra-requirements: '-c requirements/testing/minver.txt'
-            delete-font-cache: true
-            # https://github.com/matplotlib/matplotlib/issues/29844
-            pygobject-ver: '<3.52.0'
-          - os: ubuntu-22.04
-            python-version: '3.11'
-            CFLAGS: "-fno-lto"  # Ensure that disabling LTO works.
-            extra-requirements: '-r requirements/testing/extra.txt'
-            # https://github.com/matplotlib/matplotlib/issues/29844
-            pygobject-ver: '<3.52.0'
-          - os: ubuntu-22.04-arm
-            python-version: '3.12'
-            # https://github.com/matplotlib/matplotlib/issues/29844
-            pygobject-ver: '<3.52.0'
-          - name-suffix: "(Extra TeX packages)"
-            os: ubuntu-22.04
-            python-version: '3.13'
-            extra-packages: 'texlive-fonts-extra texlive-lang-cyrillic'
-            # https://github.com/matplotlib/matplotlib/issues/29844
-            pygobject-ver: '<3.52.0'
-          - name-suffix: "Free-threaded"
-            os: ubuntu-22.04
-            python-version: '3.13t'
-            # https://github.com/matplotlib/matplotlib/issues/29844
-            pygobject-ver: '<3.52.0'
           - os: ubuntu-24.04
             python-version: '3.12'
-          - os: macos-13  # This runner is on Intel chips.
-            # merge numpy and pandas install in nighties test when this runner is dropped
-            python-version: '3.11'
-          - os: macos-14  # This runner is on M1 (arm64) chips.
-            python-version: '3.12'
-            # https://github.com/matplotlib/matplotlib/issues/29732
-            pygobject-ver: '<3.52.0'
-          - os: macos-14  # This runner is on M1 (arm64) chips.
-            python-version: '3.13'
-            # https://github.com/matplotlib/matplotlib/issues/29732
-            pygobject-ver: '<3.52.0'
+            test_type: serial
 
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683  # v4.2.2
@@ -337,9 +299,21 @@ jobs:
           if [[ "${{ matrix.python-version }}" == '3.13t' ]]; then
             export PYTHON_GIL=0
           fi
-          pytest -rfEsXR -n auto \
-            --maxfail=50 --timeout=300 --durations=25 \
-            --cov-report=xml --cov=lib --log-level=DEBUG --color=yes
+          FLAGS=(
+            -rfEsXR
+            --maxfail=5
+            --timeout=300
+            --durations=25
+            --cov-report=xml
+            --cov=lib
+            --log-level=DEBUG
+            --color=yes
+          )
+          if [[ "${{ matrix.test_type }}" == 'serial' ]]; then
+              pytest "${FLAGS[@]}" -m subprocess -n 0 -vv -s
+          else
+              pytest "${FLAGS[@]}" -m 'not subprocess' -n auto
+          fi
 
       - name: Cleanup non-failed image files
         if: failure()

diff --git a/azure-pipelines.yml b/azure-pipelines.yml
@@ -49,15 +49,30 @@ stages:
       - job: Pytest
         strategy:
           matrix:
-            Windows_py311:
+            Windows_py311_parallel:
               vmImage: 'windows-2022'  # Keep one job pinned to the oldest image
               python.version: '3.11'
-            Windows_py312:
+              test.type: 'parallel'
+            Windows_py311_serial:
+              vmImage: 'windows-2022'  # Keep one job pinned to the oldest image
+              python.version: '3.11'
+              test.type: 'serial'
+            Windows_py312_parallel:
               vmImage: 'windows-latest'
               python.version: '3.12'
-            Windows_py313:
+              test.type: 'parallel'
+            Windows_py312_serial:
+              vmImage: 'windows-latest'
+              python.version: '3.12'
+              test.type: 'serial'
+            Windows_py313_parallel:
               vmImage: 'windows-latest'
               python.version: '3.13'
+              test.type: 'parallel'
+            Windows_py313_serial:
+              vmImage: 'windows-latest'
+              python.version: '3.13'
+              test.type: 'serial'
           maxParallel: 4
         pool:
           vmImage: '$(vmImage)'
@@ -120,10 +135,20 @@ stages:
 
               echo "##vso[task.setvariable variable=VS_COVERAGE_TOOL]$TOOL"
 
-              PYTHONFAULTHANDLER=1 pytest -rfEsXR -n 2 \
-                  --maxfail=50 --timeout=300 --durations=25 \
-                  --junitxml=junit/test-results.xml --cov-report=xml --cov=lib
-
+              FLAGS=(
+                -rfEsXR
+                --maxfail=50
+                --timeout=300
+                --durations=25
+                --cov-report=xml
+                --cov=lib
+                --junitxml=junit/test-results.xml
+              )
+              if [[ "$TEST_TYPE" == serial ]]; then
+                  PYTHONFAULTHANDLER=1 pytest "${FLAGS[@]}" -m subprocess -n 0
+              else
+                  PYTHONFAULTHANDLER=1 pytest "${FLAGS[@]}" -m 'not subprocess' -n 2
+              fi
               if [[ $VS_VER == 2022 ]]; then
                 "$TOOL" shutdown $SESSION_ID
                 echo "Coverage collection log"
@@ -153,7 +178,8 @@ stages:
 
           - task: PublishTestResults@2
             inputs:
-              testResultsFiles: '**/test-results.xml'
+              mergeTestResults: true
+              testResultsFiles: '**/test-results*.xml'
               testRunTitle: 'Python $(python.version)'
             condition: succeededOrFailed()
 

diff --git a/lib/matplotlib/testing/conftest.py b/lib/matplotlib/testing/conftest.py
@@ -15,6 +15,7 @@ def pytest_configure(config):
         ("markers", "backend: Set alternate Matplotlib backend temporarily."),
         ("markers", "baseline_images: Compare output against references."),
         ("markers", "pytz: Tests that require pytz to be installed."),
+        ("markers", "subprocess: Tests that start a subprocess."),
         ("filterwarnings", "error"),
         ("filterwarnings",
          "ignore:.*The py23 module has been deprecated:DeprecationWarning"),

diff --git a/lib/matplotlib/tests/test_backend_inline.py b/lib/matplotlib/tests/test_backend_inline.py
@@ -10,6 +10,7 @@
 pytest.importorskip('nbconvert')
 pytest.importorskip('ipykernel')
 pytest.importorskip('matplotlib_inline')
+pytestmark = pytest.mark.subprocess
 
 
 def test_ipynb():

diff --git a/lib/matplotlib/tests/test_backend_nbagg.py b/lib/matplotlib/tests/test_backend_nbagg.py
@@ -9,6 +9,7 @@
 nbformat = pytest.importorskip('nbformat')
 pytest.importorskip('nbconvert')
 pytest.importorskip('ipykernel')
+pytestmark = pytest.mark.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
@@ -6,6 +6,7 @@
 from matplotlib.testing import subprocess_run_for_testing
 
 
+@pytest.mark.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
@@ -22,6 +22,9 @@
 from matplotlib.testing import subprocess_run_helper as _run_helper, is_ci_environment
 
 
+pytestmark = pytest.mark.subprocess
+
+
 class _WaitForStringPopen(subprocess.Popen):
     """
     A Popen that passes flags that allow triggering KeyboardInterrupt.
@@ -33,17 +36,23 @@
         super().__init__(
             *args, **kwargs,
             # Force Agg so that each test can switch to its desired backend.
-            env={**os.environ, "MPLBACKEND": "Agg", "SOURCE_DATE_EPOCH": "0"},
-            stdout=subprocess.PIPE, universal_newlines=True)
+            env={**os.environ, "MPLBACKEND": "Agg", "SOURCE_DATE_EPOCH": "0",
+                 "PYTHONUNBUFFERED": "1"},
+            bufsize=0, stdin=subprocess.DEVNULL, stdout=subprocess.PIPE,
+            stderr=subprocess.STDOUT, universal_newlines=True)
 
     def wait_for(self, terminator):
         """Read until the terminator is reached."""
         buf = ''
         while True:
             c = self.stdout.read(1)
             if not c:
-                raise RuntimeError(
-                    f'Subprocess died before emitting expected {terminator!r}')
+                if self.poll() is None:
+                    os.sched_yield()
+                else:
+                    raise RuntimeError(
+                        f'Subprocess died before emitting expected {terminator!r}\n'
+                        f'\nSubprocess output:\n{buf}')
             buf += c
             if buf.endswith(terminator):
                 return
@@ -237,7 +246,7 @@
 
 @pytest.mark.parametrize("env", _get_testable_interactive_backends())
 @pytest.mark.parametrize("toolbar", ["toolbar2", "toolmanager"])
-@pytest.mark.flaky(reruns=3)
+#@pytest.mark.flaky(reruns=3)
 def test_interactive_backend(env, toolbar):
     if env["MPLBACKEND"] == "macosx":
         if toolbar == "toolmanager":
@@ -329,7 +338,7 @@
 
 
 @pytest.mark.parametrize("env", _thread_safe_backends)
-@pytest.mark.flaky(reruns=3)
+#@pytest.mark.flaky(reruns=3)
 def test_interactive_thread_safety(env):
     proc = _run_helper(_test_thread_impl, timeout=_test_timeout, extra_env=env)
     assert proc.stdout.count("CloseEvent") == 1
@@ -617,7 +626,7 @@
 
 @pytest.mark.parametrize("env", _blit_backends)
 # subprocesses can struggle to get the display, so rerun a few times
-@pytest.mark.flaky(reruns=4)
+#@pytest.mark.flaky(reruns=4)
 def test_blitting_events(env):
     proc = _run_helper(
         _test_number_of_draws_script, timeout=_test_timeout, extra_env=env)

diff --git a/lib/matplotlib/tests/test_basic.py b/lib/matplotlib/tests/test_basic.py
@@ -5,6 +5,8 @@
 
 from matplotlib.testing import subprocess_run_for_testing
 
+import pytest
+
 
 def test_simple():
     assert 1 + 1 == 2
@@ -28,6 +30,7 @@ def test_override_builtins():
     assert overridden <= ok_to_override
 
 
+@pytest.mark.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
@@ -21,6 +21,8 @@
 from matplotlib.text import TextPath
 from matplotlib.transforms import IdentityTransform
 
+pytestmark = pytest.mark.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
@@ -288,6 +288,7 @@ def test_fontcache_thread_safe():
     subprocess_run_helper(_test_threading, timeout=10)
 
 
+@pytest.mark.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
@@ -19,6 +19,7 @@ def test_parse_to_version_info(version_str, version_tuple):
     assert matplotlib._parse_to_version_info(version_str) == version_tuple
 
 
+@pytest.mark.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 +38,7 @@ def test_tmpconfigdir_warning(tmp_path):
         os.chmod(tmp_path, mode)
 
 
+@pytest.mark.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
@@ -245,6 +245,7 @@ def funcy(ax, x, y, z, t=None):
                          funcy.__doc__)
 
 
+@pytest.mark.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
@@ -12,6 +12,7 @@
 from matplotlib import pyplot as plt
 
 
+@pytest.mark.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
@@ -528,6 +528,7 @@ def test_rcparams_reset_after_fail():
         assert mpl.rcParams['text.usetex'] is False
 
 
+@pytest.mark.subprocess
 @pytest.mark.skipif(sys.platform != "linux", reason="Linux only")
 def test_backend_fallback_headless_invalid_backend(tmp_path):
     env = {**os.environ,
@@ -545,6 +546,7 @@ def test_backend_fallback_headless_invalid_backend(tmp_path):
             env=env, check=True, stderr=subprocess.DEVNULL)
 
 
+@pytest.mark.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 +569,7 @@ def test_backend_fallback_headless_auto_backend(tmp_path):
     assert backend.strip().lower() == "agg"
 
 
+@pytest.mark.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
@@ -16,6 +16,7 @@
 tinypages = Path(__file__).parent / 'data/tinypages'
 
 
+@pytest.mark.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
@@ -63,6 +63,7 @@ def test_unicode_characters():
         fig.canvas.draw()
 
 
+@pytest.mark.subprocess
 @needs_usetex
 def test_openin_any_paranoid():
     completed = subprocess_run_for_testing(