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gh-146369: Ensure`PYTHON_LAZY_IMPORTS=none` overrides`__lazy_modules__`#146371

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hugovk:3.15-fix-lazy-dunder-none

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gh-146369: Ensure`PYTHON_LAZY_IMPORTS=none` overrides`__lazy_modules__`#146371
hugovk wants to merge 3 commits intopython:mainfrom
hugovk:3.15-fix-lazy-dunder-none

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hugovk commentedMar 24, 2026•
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When a regular import is in__lazy_modules__,_PyEval_LazyImportName is called withlazy = 0, because this is a regular import with nolazy keyword.

The switch statement also sets it to zero, because we've set eitherPYTHON_LAZY_IMPORTS=none or-X lazy_imports=none.

However, the firstif statement checks!lazy (true) and then checks if the import is in__lazy_modules__. Well, it is, so we treat it as lazy.

Instead, we should skip thisif statement if we've set eitherPYTHON_LAZY_IMPORTS=none or-X lazy_imports=none. These should override regular imports using the backwards compatibility dunder, just like they override thelazy keyword.

With the fix, the override still works for eager:

❯hyperfine --warmup 10 --runs 20 \   "./python.exe lazy-eager.py" \   "PYTHON_LAZY_IMPORTS=normal ./python.exe lazy-eager.py" \   "PYTHON_LAZY_IMPORTS=none   ./python.exe lazy-eager.py" \   "PYTHON_LAZY_IMPORTS=all    ./python.exe lazy-eager.py" \   "./python.exe -X lazy_imports=normal lazy-eager.py" \   "./python.exe -X lazy_imports=none   lazy-eager.py" \   "./python.exe -X lazy_imports=all    lazy-eager.py"Benchmark 1: ./python.exe lazy-eager.py  Time (mean ± σ):      16.8 ms ±   0.4 ms    [User: 13.2 ms, System: 2.9 ms]  Range (min … max):    16.0 ms …  17.4 ms    20 runsBenchmark 2: PYTHON_LAZY_IMPORTS=normal ./python.exe lazy-eager.py  Time (mean ± σ):      16.9 ms ±   0.6 ms    [User: 13.2 ms, System: 3.0 ms]  Range (min … max):    15.6 ms …  17.8 ms    20 runsBenchmark 3: PYTHON_LAZY_IMPORTS=none   ./python.exe lazy-eager.py  Time (mean ± σ):     137.4 ms ±   4.2 ms    [User: 120.4 ms, System: 15.2 ms]  Range (min … max):   133.9 ms … 153.4 ms    20 runsBenchmark 4: PYTHON_LAZY_IMPORTS=all    ./python.exe lazy-eager.py  Time (mean ± σ):      17.1 ms ±   0.6 ms    [User: 13.4 ms, System: 3.0 ms]  Range (min … max):    16.0 ms …  18.1 ms    20 runsBenchmark 5: ./python.exe -X lazy_imports=normal lazy-eager.py  Time (mean ± σ):      16.7 ms ±   0.4 ms    [User: 13.2 ms, System: 2.9 ms]  Range (min … max):    15.9 ms …  17.6 ms    20 runsBenchmark 6: ./python.exe -X lazy_imports=none   lazy-eager.py  Time (mean ± σ):     139.1 ms ±   3.9 ms    [User: 121.9 ms, System: 15.2 ms]  Range (min … max):   134.9 ms … 153.4 ms    20 runsBenchmark 7: ./python.exe -X lazy_imports=all    lazy-eager.py  Time (mean ± σ):      17.0 ms ±   0.5 ms    [User: 13.3 ms, System: 3.0 ms]  Range (min … max):    16.0 ms …  18.3 ms    20 runsSummary  ./python.exe -X lazy_imports=normal lazy-eager.py ran    1.01 ± 0.03 times faster than ./python.exe lazy-eager.py    1.01 ± 0.05 times faster than PYTHON_LAZY_IMPORTS=normal ./python.exe lazy-eager.py    1.02 ± 0.04 times faster than ./python.exe -X lazy_imports=all    lazy-eager.py    1.02 ± 0.05 times faster than PYTHON_LAZY_IMPORTS=all    ./python.exe lazy-eager.py    8.23 ± 0.33 times faster than PYTHON_LAZY_IMPORTS=none   ./python.exe lazy-eager.py    8.33 ± 0.31 times faster than ./python.exe -X lazy_imports=none   lazy-eager.py

And now also for the dunder (compare the issue):

❯hyperfine --warmup 10 --runs 20 \   "./python.exe lazy-dunder.py" \   "PYTHON_LAZY_IMPORTS=normal ./python.exe lazy-dunder.py" \   "PYTHON_LAZY_IMPORTS=none   ./python.exe lazy-dunder.py" \   "PYTHON_LAZY_IMPORTS=all    ./python.exe lazy-dunder.py" \   "./python.exe -X lazy_imports=normal lazy-dunder.py" \   "./python.exe -X lazy_imports=none   lazy-dunder.py" \   "./python.exe -X lazy_imports=all    lazy-dunder.py"Benchmark 1: ./python.exe lazy-dunder.py  Time (mean ± σ):      17.4 ms ±   0.5 ms    [User: 13.6 ms, System: 3.0 ms]  Range (min … max):    16.4 ms …  18.3 ms    20 runsBenchmark 2: PYTHON_LAZY_IMPORTS=normal ./python.exe lazy-dunder.py  Time (mean ± σ):      18.2 ms ±   2.1 ms    [User: 13.9 ms, System: 3.1 ms]  Range (min … max):    16.6 ms …  25.7 ms    20 runs  Warning: Statistical outliers were detected. Consider re-running this benchmark on a quiet system without any interferences from other programs. It might help to use the '--warmup' or '--prepare' options.Benchmark 3: PYTHON_LAZY_IMPORTS=none   ./python.exe lazy-dunder.py  Time (mean ± σ):     138.9 ms ±   1.8 ms    [User: 121.8 ms, System: 15.4 ms]  Range (min … max):   134.9 ms … 141.6 ms    20 runsBenchmark 4: PYTHON_LAZY_IMPORTS=all    ./python.exe lazy-dunder.py  Time (mean ± σ):      17.7 ms ±   0.5 ms    [User: 13.7 ms, System: 3.1 ms]  Range (min … max):    16.9 ms …  18.7 ms    20 runsBenchmark 5: ./python.exe -X lazy_imports=normal lazy-dunder.py  Time (mean ± σ):      17.3 ms ±   0.5 ms    [User: 13.5 ms, System: 3.0 ms]  Range (min … max):    16.5 ms …  18.3 ms    20 runsBenchmark 6: ./python.exe -X lazy_imports=none   lazy-dunder.py  Time (mean ± σ):     138.6 ms ±   4.4 ms    [User: 121.1 ms, System: 15.5 ms]  Range (min … max):   130.6 ms … 154.4 ms    20 runsBenchmark 7: ./python.exe -X lazy_imports=all    lazy-dunder.py  Time (mean ± σ):      18.1 ms ±   0.7 ms    [User: 14.0 ms, System: 3.1 ms]  Range (min … max):    17.0 ms …  19.1 ms    20 runsSummary  ./python.exe -X lazy_imports=normal lazy-dunder.py ran    1.01 ± 0.04 times faster than ./python.exe lazy-dunder.py    1.02 ± 0.04 times faster than PYTHON_LAZY_IMPORTS=all    ./python.exe lazy-dunder.py    1.04 ± 0.05 times faster than ./python.exe -X lazy_imports=all    lazy-dunder.py    1.05 ± 0.12 times faster than PYTHON_LAZY_IMPORTS=normal ./python.exe lazy-dunder.py    8.00 ± 0.34 times faster than ./python.exe -X lazy_imports=none   lazy-dunder.py    8.02 ± 0.26 times faster than PYTHON_LAZY_IMPORTS=none   ./python.exe lazy-dunder.py

Issue:PYTHON_LAZY_IMPORTS=none ignored for__lazy_modules__ #146369

hugovk added3 commits

March 24, 2026 11:51

Failing test: lazy_imports=none or PYTHON_LAZY_IMPORTS=none should ov…

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…erride __lazy_modules__

Ensure lazy_imports=none or PYTHON_LAZY_IMPORTS=none override __lazy_…

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…modules__

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hugovk requested review fromDinoV,Yhg1s,markshannon andpablogsal ascode owners

March 24, 2026 11:17

bedevere-appbot added the awaiting core review label

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Mar 24, 2026

PYTHON_LAZY_IMPORTS=none ignored for__lazy_modules__#146369

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LGTM

bedevere-appbot added awaiting merge and removed awaiting core review labels

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pablogsal commentedMar 24, 2026

Good catch@hugovk!

vstinner reviewed

Mar 24, 2026

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Lib/test/test_lazy_import/__init__.py

		self.assertEqual(result.returncode,0,f"stderr:{result.stderr}")
		self.assertIn("EAGER",result.stdout)

		deftest_cli_lazy_imports_none_disables_dunder_lazy_modules(self):

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You can merge the two tests since they have code in common:

deftest_cli_disable_lazy_imports(self):code=textwrap.dedent("""            import sys            __lazy_modules__ = ["json"]            import json            if 'json' in sys.modules:                print("EAGER")            else:                print("LAZY")        """)# -X lazy_imports=none should override __lazy_modules__result=subprocess.run(            [sys.executable,"-X","lazy_imports=none","-c",code],capture_output=True,text=True,        )self.assertEqual(result.returncode,0,f"stderr:{result.stderr}")self.assertIn("EAGER",result.stdout)# PYTHON_LAZY_IMPORTS=none should override __lazy_modules__env=dict(os.environ,PYTHON_LAZY_IMPORTS="none")result=subprocess.run(            [sys.executable,"-c",code],capture_output=True,text=True,env=env,        )self.assertEqual(result.returncode,0,f"stderr:{result.stderr}")self.assertIn("EAGER",result.stdout)

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gh-146369: Ensure`PYTHON_LAZY_IMPORTS=none` overrides`__lazy_modules__`#146371

gh-146369: Ensure`PYTHON_LAZY_IMPORTS=none` overrides`__lazy_modules__`#146371
hugovk wants to merge 3 commits intopython:mainfrom
hugovk:3.15-fix-lazy-dunder-none

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