test: force headless Matplotlib backend for local + CI test runs

tests/base/test_transforms3d_plot.py

@@ -74,9 +74,12 @@ def test_plot(self):
         plt.close("all")
 
     @pytest.mark.skipif(
-        os.environ.get("CI") == "true"
+        plt.get_backend().lower() == "agg"
+        or os.environ.get("CI") == "true"
         or (sys.platform.startswith("darwin") and sys.version_info < (3, 11)),
-        reason="no display in CI / tkinter bug on mac",
+        reason="animation wait=True busy-loop never terminates under the "
+        "non-interactive Agg backend (no event loop to deregister the "
+        "timer callback); needs a real display",
     )
     def test_animate(self):
         tranimate(transl(1, 2, 3), repeat=False, wait=True)

tests/conftest.py

@@ -0,0 +1,14 @@
+import os
+
+# Force a non-interactive Matplotlib backend for the whole test session,
+# before any test module or package code gets a chance to import
+# matplotlib.pyplot. Several modules (geom2d, geom3d, spline, animate)
+# import pyplot at module load time, so this has to happen here, in
+# conftest.py, which pytest guarantees to load before collecting tests.
+#
+# CI already sets MPLBACKEND=Agg via the workflow env, so this mainly
+# fixes local runs, which otherwise use the platform's interactive
+# backend and pop up real windows / can hang on plt.pause(). setdefault
+# (not a hard override) leaves an escape hatch: run with
+# MPLBACKEND=MacOSX pytest ... to actually see a plot when you want to.
+os.environ.setdefault("MPLBACKEND", "Agg")

tests/test_spline.py

@@ -28,7 +28,11 @@ def test_evaluation(self):
         nt.assert_almost_equal(spline(0).A, self.control_poses[0].A)
         nt.assert_almost_equal(spline(1).A, self.control_poses[-1].A)
 
-    @pytest.mark.skipif(os.environ.get("CI") == "true", reason="no display in CI")
+    @pytest.mark.skipif(
+        plt.get_backend().lower() == "agg" or os.environ.get("CI") == "true",
+        reason="animate=True busy-waits on a timer callback that never "
+        "fires under the non-interactive Agg backend",
+    )
     def test_visualize(self):
         spline = BSplineSE3(self.control_poses)
         spline.visualize(
@@ -69,7 +73,11 @@ def test_small_delta_t(self):
             np.linspace(0, InterpSplineSE3._e, len(self.waypoints)), self.waypoints
         )
 
-    @pytest.mark.skipif(os.environ.get("CI") == "true", reason="no display in CI")
+    @pytest.mark.skipif(
+        plt.get_backend().lower() == "agg" or os.environ.get("CI") == "true",
+        reason="animate=True busy-waits on a timer callback that never "
+        "fires under the non-interactive Agg backend",
+    )
     def test_visualize(self):
         spline = InterpSplineSE3(self.times, self.waypoints)
         spline.visualize(
@@ -110,7 +118,9 @@ def test_spline_fit(self):
 
         assert fit.max_angular_error() < np.deg2rad(5.0)
         assert fit.max_angular_error() < 0.1
-        if os.environ.get("CI") != "true":
+        # animate=True busy-waits on a timer callback that never fires
+        # under the non-interactive Agg backend
+        if plt.get_backend().lower() != "agg" and os.environ.get("CI") != "true":
             spline.visualize(
                 sample_times=np.linspace(0, self.time_horizon, 100),
                 animate=True,