Do not close figures on backend switch.

anntzer · anntzer · commit 45bba0b91b6b · 2023-08-08T22:51:38.000+02:00
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/doc/api/next_api_changes/deprecations/XXXXX-AL.rst b/doc/api/next_api_changes/deprecations/XXXXX-AL.rst
@@ -0,0 +1,5 @@
+Auto-closing of figures when switching backend
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+... is deprecated.  Explicitly call ``plt.close("all")`` if necessary.  In the
+future, allowable backend switches (i.e. those that do not swap a GUI event
+loop with another one) will not close existing figures.
diff --git a/lib/matplotlib/backend_bases.py b/lib/matplotlib/backend_bases.py
@@ -45,7 +45,7 @@
 import matplotlib as mpl
 from matplotlib import (
     _api, backend_tools as tools, cbook, colors, _docstring, text,
-    _tight_bbox, transforms, widgets, get_backend, is_interactive, rcParams)
+    _tight_bbox, transforms, widgets, is_interactive, rcParams)
 from matplotlib._pylab_helpers import Gcf
 from matplotlib.backend_managers import ToolManager
 from matplotlib.cbook import _setattr_cm
@@ -2736,8 +2736,8 @@ 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/pyplot.py b/lib/matplotlib/pyplot.py
@@ -420,11 +420,16 @@ def draw_if_interactive():
     # Need to keep a global reference to the backend for compatibility reasons.
     # See https://github.com/matplotlib/matplotlib/issues/6092
     matplotlib.backends.backend = newbackend  # type: ignore[attr-defined]
+
     if not cbook._str_equal(old_backend, newbackend):
+        if get_fignums():
+            _api.warn_deprecated("3.8", message=(
+                "Auto-close()ing of figures upon backend switching is deprecated since "
+                "%(since)s and will be removed %(removal)s.  To suppress this warning, "
+                "explicitly call plt.close('all') first."))
         close("all")
 
-    # make sure the repl display hook is installed in case we become
-    # interactive
+    # Make sure the repl display hook is installed in case we become interactive.
     install_repl_displayhook()
 
 
diff --git a/lib/matplotlib/testing/conftest.py b/lib/matplotlib/testing/conftest.py
@@ -75,7 +75,9 @@ def mpl_test_settings(request):
         try:
             yield
         finally:
-            matplotlib.use(prev_backend)
+            if backend is not None:
+                plt.close("all")
+                matplotlib.use(prev_backend)
 
 
 @pytest.fixture
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
@@ -167,6 +167,7 @@ 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")
diff --git a/lib/matplotlib/tests/test_pyplot.py b/lib/matplotlib/tests/test_pyplot.py
@@ -439,7 +439,8 @@ def test_switch_backend_no_close():
     assert len(plt.get_fignums()) == 2
     plt.switch_backend('agg')
     assert len(plt.get_fignums()) == 2
-    plt.switch_backend('svg')
+    with pytest.warns(mpl.MatplotlibDeprecationWarning):
+        plt.switch_backend('svg')
     assert len(plt.get_fignums()) == 0
 
 
