Make required_interactive_framework required on FigureCanvas.

All FigureCanvas subclasses that inherit from FigureCanvasBase already
have that attribute (which may be None), so the getattr is redundant.
In practice, it would be extremely difficult to write a FigureCanvasLike
class that does not inherit from FigureCanvasBase, but to be safe, go
through a deprecation period.

Author: anntzer

doc/api/next_api_changes/deprecations/22051-AL.rst

@@ -0,0 +1,4 @@
+``FigureCanvas`` without a ``required_interactive_framework`` attribute
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Support for such canvas classes is deprecated.  Note that canvas classes which
+inherit from ``FigureCanvasBase`` always have such an attribute.

lib/matplotlib/backend_bases.py

@@ -1650,9 +1650,13 @@ def _fix_ipython_backend2gui(cls):
             # In case we ever move the patch to IPython and remove these APIs,
             # don't break on our side.
             return
-        rif = getattr(cls, "required_interactive_framework", None)
-        backend2gui_rif = {"qt": "qt", "gtk3": "gtk3", "gtk4": "gtk4",
-                           "wx": "wx", "macosx": "osx"}.get(rif)
+        backend2gui_rif = {
+            "qt": "qt",
+            "gtk3": "gtk3",
+            "gtk4": "gtk4",
+            "wx": "wx",
+            "macosx": "osx",
+        }.get(cls.required_interactive_framework)
         if backend2gui_rif:
             if _is_non_interactive_terminal_ipython(ip):
                 ip.enable_gui(backend2gui_rif)

lib/matplotlib/pyplot.py

@@ -199,8 +199,13 @@ def findobj(o=None, match=None, include_self=True):
 
 
 def _get_required_interactive_framework(backend_mod):
-    return getattr(
-        backend_mod.FigureCanvas, "required_interactive_framework", None)
+    if not hasattr(backend_mod.FigureCanvas, "required_interactive_framework"):
+        _api.warn_deprecated(
+            "3.6", name="Support for FigureCanvases without a "
+            "required_interactive_framework attribute")
+        return None
+    # Inline this once the deprecation elapses.
+    return backend_mod.FigureCanvas.required_interactive_framework
 
 
 def switch_backend(newbackend):