matplotlib · ksunden · Aug 8, 2023 · Dec 12, 2019
diff --git a/doc/api/next_api_changes/deprecations/26472-AL.rst b/doc/api/next_api_changes/deprecations/26472-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