TST: Ensure that backend_fallback is enabled during test case

jayaddison · jayaddison · commit 2b61d2bb965b · 2025-03-12T17:42:26.000Z
diff --git a/lib/matplotlib/tests/test_rcparams.py b/lib/matplotlib/tests/test_rcparams.py
@@ -539,10 +539,16 @@ def test_backend_fallback_headless_invalid_backend(tmp_path):
 
 @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
     env = {**os.environ,
            "DISPLAY": "", "WAYLAND_DISPLAY": "",
            "MPLBACKEND": "TkAgg", "MPLCONFIGDIR": str(tmp_path)}
-    # plotting should succeed using auto-backend selection in a headless environment
+
+    # allow fallback to an available interactive backend explicitly in configuration
+    rc_path = tmp_path / "matplotlibrc"
+    rc_path.write_text("backend_fallback: true")
+
+    # plotting should succeed, by falling back to use the generic agg backend
     backend = subprocess_run_for_testing(
         [sys.executable, "-c",
          "import matplotlib.pyplot;"
