Test whether it is viable to not close figures on backend switch.

anntzer · anntzer · commit c8ca4fcca838 · 2023-07-03T13:57:03.000+02:00
DO NOT MERGE.  The desired change would need a deprecation period, but
this is just to check on CI that the end state works.

Do not `close("all")` figures on (allowable) backend switches, e.g.
between qt5agg and qt5cairo, or qt5agg and notebook.

The NonGuiException message had to change, as the backend returned by
get_backend() may no longer match the actual canvas class.
diff --git a/lib/matplotlib/backend_bases.py b/lib/matplotlib/backend_bases.py
@@ -2734,8 +2734,8 @@ def show(self):
             # thus warrants a warning.
             return
         raise NonGuiException(
-            f"Matplotlib is currently using {get_backend()}, which is a "
-            f"non-GUI backend, so cannot show the figure.")
+            f"{type(self.canvas).__name__} is non-interactive, and thus cannot be "
+            f"shown")
 
     def destroy(self):
         pass
diff --git a/lib/matplotlib/tests/test_backend_bases.py b/lib/matplotlib/tests/test_backend_bases.py
@@ -85,13 +85,13 @@ def test_non_gui_warning(monkeypatch):
     with pytest.warns(UserWarning) as rec:
         plt.show()
         assert len(rec) == 1
-        assert ('Matplotlib is currently using pdf, which is a non-GUI backend'
+        assert ('FigureCanvasPdf is non-interactive, and thus cannot be shown'
                 in str(rec[0].message))
 
     with pytest.warns(UserWarning) as rec:
         plt.gcf().show()
         assert len(rec) == 1
-        assert ('Matplotlib is currently using pdf, which is a non-GUI backend'
+        assert ('FigureCanvasPdf is non-interactive, and thus cannot be shown'
                 in str(rec[0].message))
 
 
diff --git a/lib/matplotlib/tests/test_backends_interactive.py b/lib/matplotlib/tests/test_backends_interactive.py
@@ -145,6 +145,7 @@ def check_alt_backend(alt_backend):
             fig = plt.figure()
             assert (type(fig.canvas).__module__ ==
                     f"matplotlib.backends.backend_{alt_backend}")
+            plt.close("all")
 
         if importlib.util.find_spec("cairocffi"):
             check_alt_backend(backend[:-3] + "cairo")
