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GeezerVison is showing github.com — gh-129813, PEP 782: Use PyBytesWriter in bufferedio.c by vstinner · Pull Request #138954 · python/cpython

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@vstinner

@vstinner vstinner commented Sep 15, 2025 •
edited by bedevere-app Bot
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Replace PyBytes_FromStringAndSize(NULL, size) and _PyBytes_Resize() with the new public PyBytesWriter API.

  • Issue: [C API] PEP 782: Add PyBytesWriter API #129813

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@vstinner
pythongh-129813, PEP 782: Use PyBytesWriter in bufferedio.c
2ca682e 
Replace PyBytes_FromStringAndSize(NULL, size) and _PyBytes_Resize()
with the new public PyBytesWriter API.
@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 added the skip news label Sep 15, 2025
@vstinner

vstinner commented Sep 15, 2025

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Benchmark:

import pyperf
import os

file = open(__file__, "rb")
def func():
    file.seek(0)
    for _ in range(100):
        file.read1(1)

runner = pyperf.Runner()
runner.bench_func('bench', func)

file.close()

Result: Mean +- std dev: [ref] 87.0 us +- 1.5 us -> [change] 86.0 us +- 1.5 us: 1.01x faster.
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@vstinner

vstinner commented Sep 15, 2025

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cc @cmaloney
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cmaloney
cmaloney approved these changes Sep 15, 2025
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👍 looks good to me; could make the PyBytesWriter a little later and save some cleanup code (but would mean making it during the buffered lock)

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Comment thread Modules/_io/bufferedio.c
if (n == 0)
return PyBytes_FromStringAndSize(NULL, 0);
if (n == 0) {
return Py_GetConstant(Py_CONSTANT_EMPTY_BYTES);

@cmaloney cmaloney Sep 15, 2025

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This comes up fairly frequently in the I/O code, thinking of making a _Py_EMPTY_BYTES internal macro for it (separate from this PR)

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@vstinner vstinner Sep 15, 2025

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Do you mean a macro specific to the _io module?

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@cmaloney cmaloney Sep 15, 2025

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I was thinking pycore_bytesobject.h; currently shows up 24 times across the CPython source code. BufferedIO and a couple other places also use a pattern: (PyObject *)&_Py_SINGLETON(bytes_empty)

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Comment thread Modules/_io/bufferedio.c Resolved

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@vstinner
Address cmaloney's review
dd3f975 
Move PyBytesWriter_Create() call
cmaloney
cmaloney approved these changes Sep 15, 2025
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@vstinner
vstinner enabled auto-merge (squash) September 15, 2025 21:29
@vstinner
vstinner merged commit 4554486 into python:main Sep 15, 2025
@vstinner
vstinner deleted the pybyteswriter_bufferedio branch September 15, 2025 21:50
@bedevere-app bedevere-app Bot removed the awaiting core review label Sep 15, 2025
@vstinner

vstinner commented Sep 15, 2025

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Merged, thanks for the review and suggestions @cmaloney.
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@cmaloney cmaloney cmaloney approved these changes

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@vstinner @cmaloney
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