Deprecate case-insensitive properties.

anntzer · anntzer · commit bd6fe5c8debb · 2020-04-02T23:46:58.000+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
@@ -422,3 +422,8 @@ Setting the *orientation* of an ``eventplot()`` or `.EventCollection` to "none"
 or None is deprecated; set it to "horizontal" instead.  Moreover, the two
 orientations ("horizontal" and "vertical") will become case-sensitive in the
 future.
+
+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":
