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gh-90533: Implement BytesIO.peek()#150917

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marcelm wants to merge 46 commits into
python:mainfrom
marcelm:fix-issue-46375
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gh-90533: Implement BytesIO.peek()#150917
marcelm wants to merge 46 commits into
python:mainfrom
marcelm:fix-issue-46375

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@marcelm

@marcelm marcelm commented Jun 4, 2026 •
edited by bedevere-app Bot
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marcelm and others added 30 commits April 10, 2026 20:37
@marcelm
Implement BytesIO.peek()
b833b83
@blurb-it @marcelm
📜🤖 Added by blurb_it.
50a2cfb
@marcelm
Document BytesIO.peek()
eaa7672
@marcelm
Implement with the help of read_bytes()
00457ae 
This allows peek() to use the same optimization that read_bytes() has of
returning a reference to the buffer when possible (without copying).
@marcelm
Add to What’s New
882579d
@marcelm
versionadded: 3.12 -> 3.13
c1eed72
@marcelm
Remove unused variable
afc200c
@marcelm
Test tell() after peek()
79ab9a4
@marcelm
Update docs, factor out peek_bytes, semantics
b493914 
Semantic change: The default argument for peek is now size=1.
@marcelm @erlend-aasland
Update Misc/NEWS.d/next/Library/2022-04-10-20-10-59.bpo-46375.8j1ogZ.rst
2a1c85c 
Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com>
@marcelm @erlend-aasland
Update Modules/_io/bytesio.c
d398717 
Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com>
@marcelm @erlend-aasland
Update Modules/_io/bytesio.c
26d1e81 
Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com>
@marcelm @erlend-aasland
Use SemBr
9a19ff9 
Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com>
@marcelm @erlend-aasland
Update Doc/whatsnew/3.13.rst
9300ade 
Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com>
@marcelm @vstinner
Apply suggestions from code review
d214089 
Co-authored-by: Victor Stinner <vstinner@python.org>
@marcelm
Use a context manager around memio in test_peek
d6691b8
@marcelm
Add more tests for tell() after peek()
3e51adb
@marcelm
Document why size < 0 can happen
3661b65
@marcelm @erlend-aasland
Update Modules/_io/bytesio.c
cd40d77 
Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com>
@marcelm
Do not update pos if peek_bytes failed
04372bd
@marcelm
Size can be negative after truncate or seek
6b9ae8c
@marcelm
Test with size<0 and size>len(buf)
f7406f6
@marcelm
Test peek() after write()
d9528e2
@marcelm
Document BufferedReader.peek and BytesIO.peek similarly
bc8134b
@marcelm
Comment
b6ffca8
@marcelm
Make it more explicit that size is ignored
5fe5645
@marcelm
Return an empty bytes object for size=0
4126a64
@marcelm
Simplify
1ea40c2
@marcelm
Test peek(3) and peek(5)
77e04d6
@marcelm
Run clinic.py
4d2f2dd
@marcelm marcelm requested a review from AA-Turner as a code owner June 4, 2026 13:03
@bedevere-app bedevere-app Bot mentioned this pull request Jun 4, 2026
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@vstinner vstinner mentioned this pull request Jun 4, 2026
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@read-the-docs-community

read-the-docs-community Bot commented Jun 4, 2026 •
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Documentation build overview

📚 cpython-previews | 🛠️ Build #33049245 | 📁 Comparing 0be970e against main (a96cba5)

  🔍 Preview build  

18 files changed · ± 18 modified

± Modified

StanFromIreland
StanFromIreland reviewed Jun 4, 2026
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Comment thread Doc/library/io.rst Outdated
@marcelm @StanFromIreland
Update Doc/library/io.rst
3b71960 
Co-authored-by: Stan Ulbrych <stan@python.org>
@cmaloney

cmaloney commented Jun 4, 2026

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Looks like make clinic needs to be re-run and committed (lots of things can cause that)

@cmaloney

cmaloney commented Jun 4, 2026

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The "Windows (free threading)" failure looks like a flaky on main test: test.test_logging.TimedRotatingFileHandlerTest.test_rollover_based_on_st_birthtime_only also failed on macOS (free-threading)

@vstinner

vstinner commented Jun 5, 2026

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The "Windows (free threading)" failure looks like a flaky on main test: test.test_logging.TimedRotatingFileHandlerTest.test_rollover_based_on_st_birthtime_only also failed on macOS (free-threading)

I created issue gh-150960 to track the test_logging failure.

@cmaloney

cmaloney commented Jun 9, 2026

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CI all green! :)

vstinner
vstinner reviewed Jun 9, 2026
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Comment thread Doc/library/io.rst
.. method:: peek(size=0, /)

Return bytes from the current position onwards without advancing the position.
At least one byte of data is returned if not at EOF.

@vstinner vstinner Jun 9, 2026

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The size=0 case is not well documented. It's unclear to me when I read the doc if peek() or peek(0) return an empty string or something else.

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@cmaloney cmaloney cmaloney approved these changes

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io.BytesIO does not have peek()

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@marcelm @cmaloney @vstinner @StanFromIreland @emmatyping