Implement ArtistList.__[r]add__.

Fixes matplotlib#19890.

Author: QuLogic

lib/matplotlib/axes/_base.py

@@ -1363,6 +1363,16 @@ def __getitem__(self, key):
                     for artist in self._axes._children
                     if self._type_check(artist)][key]
 
+        def __add__(self, other):
+            if isinstance(other, (list, _AxesBase.ArtistList)):
+                return [*self, *other]
+            return NotImplemented
+
+        def __radd__(self, other):
+            if isinstance(other, list):
+                return other + list(self)
+            return NotImplemented
+
         def insert(self, index, item):
             _api.warn_deprecated(
                 '3.5',

lib/matplotlib/tests/test_axes.py

@@ -7104,3 +7104,7 @@ def test_artist_sublists():
                       match='modification of the Axes.lines property'):
         ax.lines[1:1] = lines[1:-2]
     assert list(ax.lines) == lines
+
+    # Adding to other lists should produce a regular list.
+    assert ax.lines + [1, 2, 3] == [*lines, 1, 2, 3]
+    assert [1, 2, 3] + ax.lines == [1, 2, 3, *lines]