Deprecate error_msg_foo helpers.

They are clearly intended for Matplotlib's internal use (e.g.
error_msg_wx sets the title of the message dialog to "Matplotlib backend
error"), and similar helpers have already been removed from other
backends.

Also correctly set the parents on the message dialogs, don't bother
setting the button on wx.MessageDialog (which already defaults to
CENTRE|OK), make the wx title also apply to other wxfoo backends, and
rename a variable in backend_wx for consistency.

Author: anntzer

doc/api/next_api_changes/deprecations/21187-AL.rst

@@ -0,0 +1,3 @@
+``backend_gtk3.error_msg_gtk`` and ``backend_wx.error_msg_wx``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+... are deprecated.

lib/matplotlib/backends/backend_gtk3.py

@@ -526,7 +526,11 @@ def on_notify_filter(*args):
         try:
             self.canvas.figure.savefig(fname, format=fmt)
         except Exception as e:
-            error_msg_gtk(str(e), parent=self)
+            dialog = Gtk.MessageDialog(
+                parent=self.canvas.get_toplevel(), message_format=str(e),
+                type=Gtk.MessageType.ERROR, buttons=Gtk.ButtonsType.OK)
+            dialog.run()
+            dialog.destroy()
 
 
 class ToolbarGTK3(ToolContainerBase, Gtk.Box):
@@ -722,6 +726,7 @@ def trigger(self, *args, **kwargs):
         clipboard.set_image(pb)
 
 
+@_api.deprecated("3.6")
 def error_msg_gtk(msg, parent=None):
     if parent is not None:  # find the toplevel Gtk.Window
         parent = parent.get_toplevel()

lib/matplotlib/backends/backend_wx.py

@@ -56,6 +56,7 @@ class __getattr__:
     }))
 
 
+@_api.deprecated("3.6")
 def error_msg_wx(msg, parent=None):
     """Signal an error condition with a popup error dialog."""
     dialog = wx.MessageDialog(parent=parent,
@@ -1153,15 +1154,15 @@ def save_figure(self, *args):
         # Fetch the required filename and file type.
         filetypes, exts, filter_index = self.canvas._get_imagesave_wildcards()
         default_file = self.canvas.get_default_filename()
-        dlg = wx.FileDialog(
+        dialog = wx.FileDialog(
             self.canvas.GetParent(), "Save to file",
             mpl.rcParams["savefig.directory"], default_file, filetypes,
             wx.FD_SAVE | wx.FD_OVERWRITE_PROMPT)
-        dlg.SetFilterIndex(filter_index)
-        if dlg.ShowModal() == wx.ID_OK:
-            path = pathlib.Path(dlg.GetPath())
+        dialog.SetFilterIndex(filter_index)
+        if dialog.ShowModal() == wx.ID_OK:
+            path = pathlib.Path(dialog.GetPath())
             _log.debug('%s - Save file path: %s', type(self), path)
-            fmt = exts[dlg.GetFilterIndex()]
+            fmt = exts[dialog.GetFilterIndex()]
             ext = path.suffix[1:]
             if ext in self.canvas.get_supported_filetypes() and fmt != ext:
                 # looks like they forgot to set the image type drop
@@ -1176,7 +1177,11 @@ def save_figure(self, *args):
             try:
                 self.canvas.figure.savefig(str(path), format=fmt)
             except Exception as e:
-                error_msg_wx(str(e))
+                dialog = wx.MessageDialog(
+                    parent=self.canvas.GetParent(), message=str(e),
+                    caption='Matplotlib error')
+                dialog.ShowModal()
+                dialog.Destroy()
 
     def draw_rubberband(self, event, x0, y0, x1, y1):
         height = self.canvas.figure.bbox.height