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GeezerVison is showing github.com — gh-129813, PEP 782: Add PyBytesWriter.overallocate by vstinner · Pull Request #138941 · python/cpython

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@vstinner

@vstinner vstinner commented Sep 15, 2025 •
edited by bedevere-app Bot
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  • Disable overallocation in _PyBytesWriter_CreateByteArray().
  • Disable overallocation in _PyBytes_FormatEx() at the last write.
  • Issue: [C API] PEP 782: Add PyBytesWriter API #129813

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@vstinner
pythongh-129813, PEP 782: Add PyBytesWriter.overallocate
f4de031 
* Disable overallocation in _PyBytesWriter_CreateByteArray().
* Disable overallocation in _PyBytes_FormatEx() at the last write.
@vstinner vstinner added the skip news label Sep 15, 2025
@bedevere-app bedevere-app Bot added the awaiting core review label Sep 15, 2025
@bedevere-app bedevere-app Bot mentioned this pull request Sep 15, 2025
[C API] PEP 782: Add PyBytesWriter API #129813
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@vstinner

vstinner commented Sep 15, 2025

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Disable overallocation in _PyBytes_FormatEx() at the last write.

Benchmark: python -m pyperf timeit -s 's=(b"a" * 254 + b"%s"); arg=b"b"*100' 's % (arg,)'.

Result: Mean +- std dev: [ref] 165 ns +- 4 ns -> [change] 147 ns +- 1 ns: 1.12x faster. It's 18 ns faster because we avoid a realloc().
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@vstinner

vstinner commented Sep 15, 2025

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@serhiy-storchaka: This change reintroduces the overallocate=0 micro-optimization in the PyBytesWriter API, but the API to disable overallocation is only available in the internal C API.
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@vstinner vstinner mentioned this pull request Sep 15, 2025
gh-129813, PEP 782: Use PyBytesWriter in utf8_encoder() #138874
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@vstinner
Simplify byteswriter_resize()
3b7b89a
@vstinner
vstinner merged commit 4e00e25 into python:main Sep 15, 2025
@vstinner
vstinner deleted the pybyteswriter_overallocate branch September 15, 2025 22:15
@bedevere-app bedevere-app Bot removed the awaiting core review label Sep 15, 2025
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@vstinner
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