Make NavigationToolbar.configure_subplots return value consistent (matplotlib#30130)

When the subplot tool already exists, then it is shown, but never
returned. This seems to be an accident.

Author: QuLogic

lib/matplotlib/backend_bases.py

@@ -3244,7 +3244,7 @@ def _update_view(self):
     def configure_subplots(self, *args):
         if hasattr(self, "subplot_tool"):
             self.subplot_tool.figure.canvas.manager.show()
-            return
+            return self.subplot_tool
         # This import needs to happen here due to circular imports.
         from matplotlib.figure import Figure
         with mpl.rc_context({"toolbar": "none"}):  # No navbar for the toolfig.

lib/matplotlib/backend_bases.pyi

@@ -475,7 +475,7 @@ class NavigationToolbar2:
     def release_zoom(self, event: Event) -> None: ...
     def push_current(self) -> None: ...
     subplot_tool: widgets.SubplotTool
-    def configure_subplots(self, *args): ...
+    def configure_subplots(self, *args: Any) -> widgets.SubplotTool: ...
     def save_figure(self, *args) -> str | None | object: ...
     def update(self) -> None: ...
     def set_history_buttons(self) -> None: ...

lib/matplotlib/tests/test_backend_qt.py

@@ -213,7 +213,9 @@ def set_device_pixel_ratio(ratio):
 def test_subplottool():
     fig, ax = plt.subplots()
     with mock.patch("matplotlib.backends.qt_compat._exec", lambda obj: None):
-        fig.canvas.manager.toolbar.configure_subplots()
+        tool = fig.canvas.manager.toolbar.configure_subplots()
+        assert tool is not None
+        assert tool == fig.canvas.manager.toolbar.configure_subplots()
 
 
 @pytest.mark.backend('QtAgg', skip_on_importerror=True)

lib/matplotlib/tests/test_backend_tk.py

@@ -168,7 +168,9 @@ def test_never_update():
     plt.show(block=False)
 
     plt.draw()  # Test FigureCanvasTkAgg.
-    fig.canvas.toolbar.configure_subplots()  # Test NavigationToolbar2Tk.
+    tool = fig.canvas.toolbar.configure_subplots()  # Test NavigationToolbar2Tk.
+    assert tool is not None
+    assert tool == fig.canvas.toolbar.configure_subplots()  # Tool is reused internally.
     # Test FigureCanvasTk filter_destroy callback
     fig.canvas.get_tk_widget().after(100, plt.close, fig)