swev-id: matplotlib__matplotlib-23299 Preserve figures when resolving auto backend

[casey-brooks]

Summary

• prevent RcParams.__getitem__("backend") from clearing existing figures when rcParams['backend'] is the auto sentinel and pyplot already picked a backend
• capture the active backend from matplotlib.backends.backend instead of re-invoking switch_backend() in that state
• add a subprocess regression test that reproduces the original Gcf.figs clearing and now verifies the figures remain registered

Reproduction Steps

import matplotlib.pyplot as plt
from matplotlib import get_backend, rc_context

with rc_context():
    plt.figure()

before = f"{id(plt._pylab_helpers.Gcf)} {plt._pylab_helpers.Gcf.figs!r}"
get_backend()
after = f"{id(plt._pylab_helpers.Gcf)} {plt._pylab_helpers.Gcf.figs!r}"

assert before == after, '\n' + before + '\n' + after

Observed Failure (pre-fix)

AssertionError:
94453354309744 OrderedDict([(1, <matplotlib.backends.backend_qt.FigureManagerQT object at 0x...>)])
94453354309744 OrderedDict()

The fix avoids calling switch_backend() once pyplot already resolved a backend, so the active figures stay intact. Fixes #25.

Testing

• MPLBACKEND=Agg pytest lib/matplotlib/tests/test_backends_interactive.py -k get_backend_preserves_figures -vv

Testing Notes

• Installed build tooling via nix profile add ... python310 pip gcc freetype pkg-config zlib gnumake
• Created a virtualenv, installed editable Matplotlib, and pulled in requirements/testing/all.txt
• Downgraded numpy to 1.26.4 and pyparsing to 3.1.1 per project guidance
• Set LD_LIBRARY_PATH to include the Nix GCC runtime libraries before running pytest

[casey-brooks]

Test & Lint Summary

• MPLBACKEND=Agg pytest lib/matplotlib/tests/test_backends_interactive.py -k get_backend_preserves_figures -vv (1 passed, 0 failed, 0 skipped)
• flake8 lib/matplotlib/__init__.py lib/matplotlib/tests/test_backends_interactive.py (no issues)

[noa-lucent]

Changes look good. Thanks!