Sample code for: Python 3.15 Preview: Lazy Imports - #808
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Sample code for: Python 3.15 Preview: Lazy Imports#808stephengruppetta wants to merge 2 commits into
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August 21, 2026 10:18
bench.py ignored the subprocess return code, so a script that crashed
was timed as if it had succeeded. A reader whose python isn't 3.15
would have seen cli_lazy.py die with a SyntaxError and bench.py report
it as a 2.5x speedup:
$ python3.14 bench.py cli_lazy.py --help
cli_lazy.py: 50 ms (best of 10)
That's the same silent, plausible-looking wrongness the tutorial warns
about in its own side-effects section. bench.py now checks the return
code, surfaces the captured stderr, and refuses to run at all below
3.15.
Also note in the README which files raise on purpose, and that a pyenv
build needs Tk headers or cli_eager.py won't start.
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Sample code for the Real Python tutorial Python 3.15 Preview: Lazy Imports.
Standard library only, but it needs Python 3.15 — most files use the
lazykeyword from PEP 810, which is aSyntaxErroron anything earlier. Every file here was run against a CPython 3.15.0rc1 build, and the tutorial quotes its real output.Layout
noisy_module.py,probe.pyshapes.py,partial.pybadfunc.pylazyIsn't Allowedreport_cli/type_checking_guard.py,lazy_annotation.pyif TYPE_CHECKINGDancefail.pycircular/bridge.py,allmode.pyreport_cli/holds three versions of the same CLI:cli_eager.py,cli_lazy.py(fivelazykeywords, nothing else changed), andcli_too_lazy.py, which also defers the two plugin imports and so silently empties the format registry.bench.pytimes any of them, and a--load-allflag reads every deferred name so you can check that deferral costs nothing once the modules are used.circular/has four self-contained folders:eager/fails,lazy/is fixed by deferring one side, andinit_eager/+init_lazy/both fail with the sameImportErrorbecause the cycle needs a value during module initialization.One change outside the new folder
pyproject.tomladdspython315-lazy-importsto ruff'sexcludelist. Two reasons:lazykeyword.ruff format --checkfails withinvalid-syntax: Simple statements must be separated by newlines or semicolonson any file using it, which would break CI.CI passes locally with the exclusion in place:
ruff format --check,ruff check, anddircheck.pyare all green.