8000 FIX: make safe to add / remove artists during ArtistList iteration by tacaswell · Pull Request #22229 · matplotlib/matplotlib · GitHub
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FIX: make safe to add / remove artists during ArtistList iteration #22229

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Merged
merged 3 commits into from
Jan 27, 2022
Merged

FIX: make safe to add / remove artists during ArtistList iteration #22229

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a814673
FIX: make safe to add / remove artists during ArtistList iteration
tacaswell Jan 14, 2022
c6b0f25
DOC: add behaviour change note justify this API choice
tacaswell Jan 15, 2022
fe511c5
DOC: clarify wording
tacaswell Jan 27, 2022
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18 changes: 18 additions & 0 deletions doc/api/next_api_changes/behavior/22229-TAC.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,18 @@
AritistList proxies copy contents on iteration
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

When iterating over the contents of the the dynamically generated proxy lists
for the Artist-type accessors (see :ref:`Behavioural API Changes 3.5 - Axes
children combined`), a copy of the contents is made. This ensure that artists
can safely be added or removed from the Axes while iterating over their children.

This is a departure from the expected behavior of mutable iterable data types
in Python -- iterating over list while mutating it has surprising consequences
and dictionaries will error if they change size during iteration.
Because all of the accessors are filtered views of the same underlying list, it
is possible for seemingly unrelated changes, such as removing a Line, to affect
the iteration over any of the other accessors. In this case, we have opted to
make a copy of the relevant children before yielding them to the user.

This change is also consistent with our plan to make these accessors immutable
in Matplotlib 3.7.
2 changes: 1 addition & 1 deletion lib/matplotlib/axes/_base.py
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Expand Up @@ -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

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4 changes: 4 additions & 0 deletions lib/matplotlib/tests/test_axes.py
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Expand Up @@ -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"
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0