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

anntzer · anntzer · commit 48e59598df27 · 2019-12-12T12:46:33.000+01:00
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
@@ -2563,8 +2563,8 @@ def show(self):
         # This should be overridden in GUI backends.
         if cbook._get_running_interactive_framework() != "headless":
             raise NonGuiException(
-                f"Matplotlib is currently using {get_backend()}, which is "
-                f"a non-GUI backend, so cannot show the figure.")
+                f"{type(self.canvas).__name__} is non-interactive, and thus "
+                f"cannot be shown")
 
     def destroy(self):
         pass
diff --git a/lib/matplotlib/pyplot.py b/lib/matplotlib/pyplot.py
@@ -190,7 +190,6 @@ def switch_backend(newbackend):
     newbackend : str
         The name of the backend to use.
     """
-    close("all")
 
     if newbackend is rcsetup._auto_backend_sentinel:
         # Don't try to fallback on the cairo-based backends as they each have
diff --git a/lib/matplotlib/tests/test_backend_bases.py b/lib/matplotlib/tests/test_backend_bases.py
@@ -66,13 +66,13 @@ 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
@@ -88,6 +88,7 @@ def check_alt_backend(alt_backend):
         assert_equal(
             type(fig.canvas).__module__,
             "matplotlib.backends.backend_{}".format(alt_backend))
+        plt.close("all")
 
     if importlib.util.find_spec("cairocffi"):
         check_alt_backend(backend[:-3] + "cairo")
