Restore matplotlib backend after HoloViews matplotlib plot

test/test_plot.py

@@ -229,3 +229,31 @@ 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_restored_after_switch(monkeypatch):
+    """Regression test for #1537: switching HoloViews to matplotlib must restore
+    the Matplotlib backend captured beforehand.
+
+    A bare pytest can't reproduce the Jupyter inline-hook flip, so we simulate
+    hv.extension flipping the backend and assert assign() restores it.
+    """
+    import holoviews as hv
+
+    from uxarray.plot.utils import HoloviewsBackend
+
+    original = matplotlib.get_backend()
+    try:
+        matplotlib.use("svg")  # the "user's" backend before plotting
+
+        # Simulate hv.extension("matplotlib") flipping the active backend to agg.
+        monkeypatch.setattr(hv.Store, "current_backend", "bokeh", raising=False)
+        monkeypatch.setattr(hv, "extension", lambda *a, **k: matplotlib.use("agg"))
+
+        be = HoloviewsBackend()
+        be.assign("matplotlib")
+
+        # assign() must restore the backend that was active before the switch.
+        assert matplotlib.get_backend() == "svg"
+    finally:
+        matplotlib.use(original)

uxarray/plot/utils.py

@@ -2,36 +2,37 @@
 
 
 class HoloviewsBackend:
-    """Utility class to compare and set a HoloViews plotting backend for
-    visualization."""
+    """Compare and set the HoloViews plotting backend."""
 
     def __init__(self):
         self.matplotlib_backend = None
 
     def assign(self, backend: str):
-        """Assigns a backend for use with HoloViews visualization.
-
-        Parameters
-        ----------
-        backend : str
-            Plotting backend to use, one of 'matplotlib', 'bokeh'
-        """
-
-        if self.matplotlib_backend is None:
-            import matplotlib as mpl
-
-            self.matplotlib_backend = mpl.get_backend()
-
+        """Assign a HoloViews backend, one of 'matplotlib', 'bokeh'."""
         if backend not in ["bokeh", "matplotlib", None]:
             raise ValueError(
                 f"Unsupported backend. Expected one of ['bokeh', 'matplotlib'], but received {backend}"
             )
         if backend is not None and backend != hv.Store.current_backend:
-            # only call hv.extension if it needs to be changed
+            import matplotlib as mpl
+
+            # Capture the live backend now (not once at init) so a backend the
+            # user set later is what we restore.
+            self.matplotlib_backend = mpl.get_backend()
             hv.extension(backend)
 
+            if backend == "matplotlib":
+                # hv.extension("matplotlib") switches the active Matplotlib
+                # backend (e.g. to agg in Jupyter), breaking subsequent native
+                # matplotlib/xarray .plot() calls. HoloViews renders through
+                # hv.Store.current_backend, so restoring is safe. See #1537.
+                self.reset_mpl_backend()
+
     def reset_mpl_backend(self):
-        """Resets the default backend for the ``matplotlib`` module."""
+        """Switch Matplotlib back to the backend captured before the last switch."""
+        if self.matplotlib_backend is None:
+            return
+
         import matplotlib as mpl
 
         mpl.use(self.matplotlib_backend)