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reduce gc_collect_harder default to 1 on CPython #14441

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reduce gc_collect_harder default to 1 on CPython #14441

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unraisableexception: reduce gc_collect_harder default to 1 on CPython
miketheman May 5, 2026
07255a2
Merge branch 'main' into miketheman/speed-up-test-sutie
miketheman May 8, 2026
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unraisableexception: reduce gc_collect_harder default to 1 on CPython 
The 5-iteration default was borrowed from the Trio project, where it was
determined empirically to handle PyPy's object resurrection behavior: on
PyPy, objects like coroutines can survive GC rounds because executing their
__del__ can resurrect them.

On CPython, reference counting frees most objects immediately. One GC pass
is sufficient to handle reference cycles, as confirmed by all
test_unraisableexception tests passing (including the refcycle variants).

Use 1 pass on CPython and retain 5 on PyPy.

Signed-off-by: Mike Fiedler <miketheman@gmail.com>
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miketheman committed May 5, 2026
commit 5efe979d2bec0b92ef176bc5edddbe7b0e74a31b
11 changes: 8 additions & 3 deletions src/_pytest/unraisableexception.py
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Expand Up @@ -86,9 +86,14 @@ def collect_unraisable(config: Config) -> None:
def cleanup(
*, config: Config, prev_hook: Callable[[sys.UnraisableHookArgs], object]
) -> None:
# A single collection doesn't necessarily collect everything.
# Constant determined experimentally by the Trio project.
gc_collect_iterations = config.stash.get(gc_collect_iterations_key, 5)
# On PyPy, objects (e.g. coroutines) can survive GC rounds because executing
# their __del__ can resurrect them. The Trio project determined experimentally
# that 5 passes are needed on PyPy to flush everything. On CPython, reference
# counting handles most cleanup immediately, so 1 pass is sufficient.
_default_gc_collect_iterations = 5 if hasattr(sys, "pypy_version_info") else 1
gc_collect_iterations = config.stash.get(
gc_collect_iterations_key, _default_gc_collect_iterations
)
try:
try:
gc_collect_harder(gc_collect_iterations)
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