TST: add headless-fallback success test case, and rename existing failure case

jayaddison · jayaddison · commit baf4c3f99980 · 2025-03-11T12:47:31.000Z
diff --git a/lib/matplotlib/tests/test_rcparams.py b/lib/matplotlib/tests/test_rcparams.py
@@ -521,10 +521,11 @@ def test_rcparams_reset_after_fail():
 
 
 @pytest.mark.skipif(sys.platform != "linux", reason="Linux only")
-def test_backend_fallback_headless(tmp_path):
+def test_backend_fallback_headless_invalid_backend(tmp_path):
     env = {**os.environ,
            "DISPLAY": "", "WAYLAND_DISPLAY": "",
            "MPLBACKEND": "", "MPLCONFIGDIR": str(tmp_path)}
+    # plotting should fail with the tkagg backend selected in a headless environment
     with pytest.raises(subprocess.CalledProcessError):
         subprocess_run_for_testing(
             [sys.executable, "-c",
@@ -536,6 +537,20 @@ def test_backend_fallback_headless(tmp_path):
             env=env, check=True, stderr=subprocess.DEVNULL)
 
 
+@pytest.mark.skipif(sys.platform != "linux", reason="Linux only")
+def test_backend_fallback_headless_auto_backend(tmp_path):
+    env = {**os.environ,
+           "DISPLAY": "", "WAYLAND_DISPLAY": "",
+           "MPLBACKEND": "", "MPLCONFIGDIR": str(tmp_path)}
+    # plotting should succeed using auto-backend selection in a headless environment
+    subprocess_run_for_testing(
+        [sys.executable, "-c",
+         "import matplotlib.pyplot;"
+         "matplotlib.pyplot.plot(42);"
+         ],
+        env=env, check=True, stderr=subprocess.DEVNULL)
+
+
 @pytest.mark.skipif(
     sys.platform == "linux" and not _c_internal_utils.xdisplay_is_valid(),
     reason="headless")
