swev-id: matplotlib__matplotlib-24627 - detach artist parents after cla/clf

[casey-brooks]

Summary

• ensure Axes.clear() removes child artists via remove() so previously attached artists drop their axes
• update Figure.clear() to remove axes and figure-level artists before resetting lists so .axes/.figure references are cleared
• add Agg regression tests for cla() and clf() covering legends, images, and a colorbar-backed axes

Observed failure (before fix)

============================= test session starts ==============================
platform linux -- Python 3.10.19, pytest-9.0.2, pluggy-1.6.0
rootdir: /workspace/matplotlib
configfile: pytest.ini
plugins: timeout-2.4.0, rerunfailures-16.1, xvfb-3.1.1, cov-7.0.0, xdist-3.8.0
collected 2 items

lib/matplotlib/tests/test_artist.py FF                                   [100%]

=================================== FAILURES ===================================
_________________________ test_cla_unsets_artist_axes __________________________

    @pytest.mark.backend("Agg")
    def test_cla_unsets_artist_axes():
        fig, ax = plt.subplots()
        line, = ax.plot([0, 1], [0, 1])
        patch = ax.add_patch(mpatches.Rectangle((0, 0), 1, 1))
        image = ax.imshow(np.arange(4).reshape(2, 2))
        text = ax.text(0.5, 0.5, "hi")
        collection = ax.scatter([0], [0])
        legend = ax.legend([line], ["line"])
    
        ax.cla()
    
        for artist in (line, patch, image, text, collection, legend):
>           assert artist.axes is None
E           assert <Axes: > is None
E            +  where <Axes: > = <matplotlib.lines.Line2D object at 0xffff55daeb60>.axes

lib/matplotlib/tests/test_artist.py:215: AssertionError
------------------------------ Captured log setup ------------------------------
WARNING  matplotlib.testing:__init__.py:38 Could not set locale to English/United States. Some date-related tests may fail.
___________________ test_clf_unsets_figure_and_axes_parents ____________________

    @pytest.mark.backend("Agg")
    def test_clf_unsets_figure_and_axes_parents():
        fig, ax = plt.subplots()
        line, = ax.plot([0, 1], [0, 1])
        image = ax.imshow(np.arange(4).reshape(2, 2))
        fig_text = fig.text(0.5, 0.5, "hi")
        fig_legend = fig.legend([line], ["line"])
        colorbar = fig.colorbar(image, ax=ax)
    
        fig.clf()
    
        assert fig.axes == []
>       assert line.axes is None
E       assert <Axes: > is None
E        +  where <Axes: > = <matplotlib.lines.Line2D object at 0xffff553afd60>.axes

lib/matplotlib/tests/test_artist.py:230: AssertionError
------------------------------ Captured log setup ------------------------------
WARNING  matplotlib.testing:__init__.py:38 Could not set locale to English/United States. Some date-related tests may fail.
=========================== short test summary info ============================
FAILED lib/matplotlib/tests/test_artist.py::test_cla_unsets_artist_axes - ass...
FAILED lib/matplotlib/tests/test_artist.py::test_clf_unsets_figure_and_axes_parents
============================== 2 failed in 1.12s ===============================

Reproduction

import numpy as np
import matplotlib
matplotlib.use("Agg")
from matplotlib import pyplot as plt
from matplotlib import patches

fig, ax = plt.subplots()
line, = ax.plot([0, 1], [0, 1])
rect = patches.Rectangle((0, 0), 1, 1)
ax.add_patch(rect)
legend = ax.legend([line], ["line"])
ax.cla()
assert line.axes is None
assert rect.axes is None
assert legend.axes is None

fig, ax = plt.subplots()
line, = ax.plot([0, 1], [0, 1])
im = ax.imshow(np.arange(4).reshape(2, 2))
fig_text = fig.text(0.5, 0.5, "hi")
fig_legend = fig.legend([line], ["line"])
colorbar = fig.colorbar(im, ax=ax)
fig.clf()
assert fig.axes == []
assert line.axes is None
assert fig_text.figure is None
assert fig_legend.figure is None
assert colorbar.ax.figure is None

Edge Cases & Verification

• Handles legends, containers, and colorbar axes so deparented artists drop .axes/.figure references.
• Tests: LD_LIBRARY_PATH=/root/.nix-profile/lib:/nix/store/qipd93x9gjyiygqk673rd2ssnf8y7jj0-gcc-14.3.0-lib/lib:/nix/store/f8w1i7yisixb9hivzbk0l4ixmf67fjqr-gcc-14.3.0-libgcc/lib:/nix/store/62yvg9afjvmwa8z17arn4giqmjzg2xd2-glibc-2.40-66/lib PYTHONPATH=/workspace/matplotlib/lib MPLBACKEND=Agg .venv/bin/pytest lib/matplotlib/tests/test_artist.py::test_cla_unsets_artist_axes lib/matplotlib/tests/test_artist.py::test_clf_unsets_figure_and_axes_parents
• Lint: .venv/bin/flake8 lib/matplotlib/axes/_base.py lib/matplotlib/figure.py lib/matplotlib/tests/test_artist.py

[noa-lucent]

[major] lib/matplotlib/axes.py:L4865 rewrites back to the pre-3.x implementation. The current signature is . Dropping the flexible positional handling, horizontal stems, and the LineCollection fast path is a huge backwards-incompatible regression and will break real callers and our own tests. Please keep the existing implementation and only change the minimal code needed for the fix.

[noa-lucent]

[major] The regression noted previously persists. lib/matplotlib/axes.py still replaces the current Axes.stem implementation with the legacy pre-3.x version, removing the modern signature (def stem(self, *args, linefmt=None, markerfmt=None, basefmt=None, bottom=0, label=None, use_line_collection=True, orientation='vertical')), the horizontal orientation support, and the LineCollection fast path. This is a breaking change for existing callers and fails our Agg tests. Please restore the modern implementation and apply only the minimal changes needed to clear artist parents.

[noa-lucent]

Thanks for addressing the regression risk. Clearing now calls remove() on the axes’ legend and children before resetting the bookkeeping lists, and Figure.clear removes axes and figure-level artists via their remove() hooks, so attached artists drop their axes/figure references. The new Agg regression tests cover the CLA/CLF scenarios that previously failed. Looks good to me.