Restore matplotlib backend after HoloViews matplotlib plot

test/test_plot.py

@@ -229,3 +229,29 @@ def test_to_raster_pixel_ratio(gridpath, r1, r2):
     f = r2 / r1
     d = np.array(raster2.shape) - f * np.array(raster1.shape)
     assert (d >= 0).all() and (d <= f - 1).all()
+
+
+def test_matplotlib_backend_does_not_clobber_mpl_state(gridpath):
+    """Regression test for #1537.
+
+    ``plot(backend="matplotlib")`` calls ``hv.extension("matplotlib")`` which
+    switches the active Matplotlib backend and breaks subsequent native
+    Matplotlib/xarray ``.plot()`` calls. UXarray should restore the original
+    Matplotlib backend so plain Matplotlib plotting still works afterwards.
+    """
+    mesh_path = gridpath("mpas", "QU", "oQU480.231010.nc")
+    uxds = ux.open_dataset(mesh_path, mesh_path)
+
+    backend_before = matplotlib.get_backend()
+
+    # UXarray plot using the matplotlib backend (the trigger in #1537)
+    uxds["bottomDepth"].plot(backend="matplotlib")
+
+    # The active matplotlib backend must be restored.
+    assert matplotlib.get_backend() == backend_before
+
+    # A subsequent plain xarray/matplotlib plot must still work without error.
+    fig, ax = plt.subplots()
+    xr.DataArray(np.random.rand(5, 5), dims=["y", "x"], name="plain").plot(ax=ax)
+    assert len(ax.collections) + len(ax.images) > 0
+    plt.close(fig)

uxarray/plot/utils.py

@@ -30,8 +30,24 @@ def assign(self, backend: str):
             # only call hv.extension if it needs to be changed
             hv.extension(backend)
 
+            if backend == "matplotlib":
+                # ``hv.extension("matplotlib")`` switches the active Matplotlib
+                # backend (e.g. to ``agg`` in Jupyter), which clobbers the
+                # IPython inline display hook. This silently breaks any
+                # subsequent native ``matplotlib``/``xarray`` ``.plot()`` calls,
+                # which render nothing. HoloViews objects still display because
+                # they render through ``hv.Store.current_backend`` rather than
+                # the active Matplotlib backend, so restoring the original
+                # backend here is safe and fixes downstream plotting. See
+                # https://github.com/UXARRAY/uxarray/issues/1537
+                self.reset_mpl_backend()
+
     def reset_mpl_backend(self):
-        """Resets the default backend for the ``matplotlib`` module."""
+        """Restores the original ``matplotlib`` backend (and its IPython inline
+        display hook) that was active before a HoloViews backend switch."""
+        if self.matplotlib_backend is None:
+            return
+
         import matplotlib as mpl
 
         mpl.use(self.matplotlib_backend)