FIX: make safe to add / remove artists during ArtistList iteration

If you remove artists during iteration some of the artists will be skipped due
to the underlying list being mutated.

Author: tacaswell

lib/matplotlib/axes/_base.py

@@ -1366,7 +1366,7 @@ def __len__(self):
                        for artist in self._axes._children)
 
         def __iter__(self):
-            for artist in self._axes._children:
+            for artist in list(self._axes._children):
                 if self._type_check(artist):
                     yield artist
 

lib/matplotlib/tests/test_axes.py

@@ -7360,6 +7360,10 @@ def test_artist_sublists():
     assert ax.lines + [1, 2, 3] == [*lines, 1, 2, 3]
     assert [1, 2, 3] + ax.lines == [1, 2, 3, *lines]
 
+    for ln in ax.lines:
+        ln.remove()
+    assert len(ax.lines) == 0
+
 
 def test_empty_line_plots():
     # Incompatible nr columns, plot "nothing"