TST: always force the SETUPTOOLS_SCM version in test subprocesses (#30947)

tacaswell · web-flow · commit 64cda2425218 · 2026-01-12T08:57:06.000+01:00
* TST: always force the SETUPTOOLS_SCM version in test subprocesses

This will reduce the number of (sub)subprocess calls we make by at least one per
subprocess test.  This may help reduce resource contention on CI that causes
flaky tests.
diff --git a/lib/matplotlib/testing/__init__.py b/lib/matplotlib/testing/__init__.py
@@ -151,13 +151,26 @@ def subprocess_run_helper(func, *args, timeout, extra_env=None):
             f"_module = importlib.util.module_from_spec(_spec);"
             f"_spec.loader.exec_module(_module);"
             f"_module.{target}()",
-            *args
+            *args,
         ],
-        env={**os.environ, "SOURCE_DATE_EPOCH": "0", **(extra_env or {})},
-        timeout=timeout, check=True,
+        env={
+            **os.environ,
+            "SOURCE_DATE_EPOCH": "0",
+            # subprocess_run_helper sets SOURCE_DATE_EPOCH=0 above, so for a dirty tree,
+            # the version will have the date 19700101 which breaks pickle tests with a
+            # warning if the working tree is dirty.
+            #
+            # This will also avoid at least one additional subprocess call for
+            # setuptools-scm query git, so we tell the subprocess what version
+            # to report as the test process.
+            "SETUPTOOLS_SCM_PRETEND_VERSION_FOR_MATPLOTLIB": mpl.__version__,
+            **(extra_env or {}),
+        },
+        timeout=timeout,
+        check=True,
         stdout=subprocess.PIPE,
         stderr=subprocess.PIPE,
-        text=True
+        text=True,
     )
     return proc
 
diff --git a/lib/matplotlib/tests/test_pickle.py b/lib/matplotlib/tests/test_pickle.py
@@ -150,15 +150,7 @@ def test_pickle_load_from_subprocess(fig_test, fig_ref, tmp_path):
     proc = subprocess_run_helper(
         _pickle_load_subprocess,
         timeout=60,
-        extra_env={
-            "PICKLE_FILE_PATH": str(fp),
-            "MPLBACKEND": "Agg",
-            # subprocess_run_helper will set SOURCE_DATE_EPOCH=0, so for a dirty tree,
-            # the version will have the date 19700101. As we aren't trying to test the
-            # version compatibility warning, force setuptools-scm to use the same
-            # version as us.
-            "SETUPTOOLS_SCM_PRETEND_VERSION_FOR_MATPLOTLIB": mpl.__version__,
-        },
+        extra_env={"PICKLE_FILE_PATH": str(fp), "MPLBACKEND": "Agg"},
     )
 
     loaded_fig = pickle.loads(ast.literal_eval(proc.stdout))
