pytester: patch GC_COLLECT_ITERATIONS to speed up the test suite

Because `pytester.runpytest()` executes the full session cycle (including `pytest_unconfigure`), it was calling `gc.collect()` in a loop multiple times—even for small, fast tests. This significantly increased the total test suite runtime.

To optimize performance, pytester now patches `GC_COLLECT_ITERATIONS` to skip `gc.collect()` entirely, matching the behavior before pytest-dev#12958.

Locally the test suite runtime improved dramatically, dropping from 425s to 160s.

Fixes pytest-dev#13482.

Author: nicoddemus

src/_pytest/pytester.py

@@ -706,6 +706,13 @@ def __init__(
         # Do not use colors for inner runs by default.
         mp.setenv("PY_COLORS", "0")
 
+        # Disable persistent gc.collection: pytester executes a full pytest section, including calling
+        # pytest_unconfigure().
+        # Disable gc.collect() in a loop as it causes a serious slowdown (#13482).
+        from _pytest import unraisableexception
+
+        mp.setattr(unraisableexception, "GC_COLLECT_ITERATIONS", 0)
+
     @property
     def path(self) -> Path:
         """Temporary directory path used to create files/run tests from, etc."""

src/_pytest/unraisableexception.py

@@ -24,10 +24,14 @@
     from exceptiongroup import ExceptionGroup
 
 
+# This constant was determined experimentally by the Trio project.
+# Patched by pytester.
+GC_COLLECT_ITERATIONS = 5
+
+
 def gc_collect_harder() -> None:
-    # A single collection doesn't necessarily collect everything.
-    # Constant determined experimentally by the Trio project.
-    for _ in range(5):
+    """Call gc.collect() multiple times, a single collection doesn't necessarily collect everything."""
+    for _ in range(GC_COLLECT_ITERATIONS):
         gc.collect()
 
 

testing/test_unraisableexception.py

@@ -6,6 +6,7 @@
 import sys
 from unittest import mock
 
+from _pytest import unraisableexception
 from _pytest.pytester import Pytester
 import pytest
 
@@ -238,7 +239,10 @@ def _disable_gc() -> Generator[None]:
         _set_gc_state(enabled=was_enabled)
 
 
-def test_refcycle_unraisable(pytester: Pytester) -> None:
+def test_refcycle_unraisable(
+    pytester: Pytester, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    monkeypatch.setattr(unraisableexception, "GC_COLLECT_ITERATIONS", 5)
     # see: https://github.com/pytest-dev/pytest/issues/10404
     pytester.makepyfile(
         test_it="""
@@ -267,7 +271,10 @@ def test_it():
 
 
 @pytest.mark.filterwarnings("default::pytest.PytestUnraisableExceptionWarning")
-def test_refcycle_unraisable_warning_filter(pytester: Pytester) -> None:
+def test_refcycle_unraisable_warning_filter(
+    pytester: Pytester, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    monkeypatch.setattr(unraisableexception, "GC_COLLECT_ITERATIONS", 5)
     # note that the host pytest warning filter is disabled and the pytester
     # warning filter applies during config teardown of unraisablehook.
     # see: https://github.com/pytest-dev/pytest/issues/10404
@@ -298,7 +305,10 @@ def test_it():
 
 
 @pytest.mark.filterwarnings("default::pytest.PytestUnraisableExceptionWarning")
-def test_create_task_raises_unraisable_warning_filter(pytester: Pytester) -> None:
+def test_create_task_raises_unraisable_warning_filter(
+    pytester: Pytester, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    monkeypatch.setattr(unraisableexception, "GC_COLLECT_ITERATIONS", 5)
     # note that the host pytest warning filter is disabled and the pytester
     # warning filter applies during config teardown of unraisablehook.
     # see: https://github.com/pytest-dev/pytest/issues/10404