Deprecate case-insensitive properties.

anntzer · anntzer · commit 7515f4f6ad45 · 2020-03-31T22:40:27.000+02:00
As a real case, incorrectly passing "axA" instead of "axesA" to
ConnectionPatch currently triggers a slightly confusing warning
regarding the non-existence of the "axa" property.
diff --git a/doc/api/next_api_changes/deprecations.rst b/doc/api/next_api_changes/deprecations.rst
@@ -415,3 +415,8 @@ parent figure, though they can be later removed with ``ax.remove()``.
 
 ``.BboxBase.inverse_transformed`` is deprecated (call `.BboxBase.transformed`
 on the `~.Transform.inverted()` transform instead).
+
+Case-insensitive properties
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Support for upper or mixed-case property names in `.Artist.set` and
+`.Artist.update` is deprecated.  Pass properties in lowercase.
diff --git a/lib/matplotlib/artist.py b/lib/matplotlib/artist.py
@@ -980,7 +980,12 @@ def update(self, props):
         ret = []
         with cbook._setattr_cm(self, eventson=False):
             for k, v in props.items():
-                k = k.lower()
+                if k != k.lower():
+                    cbook.warn_deprecated(
+                        "3.3", message="Case-insensitive properties were "
+                        "deprecated in %(since)s and support will be removed "
+                        "%(removal)s")
+                    k = k.lower()
                 # White list attributes we want to be able to update through
                 # art.update, art.set, setp.
                 if k == "axes":
