gh-90533: Implement BytesIO.peek() by marcelm · Pull Request #30808 · python/cpython · GitHub

Most changes to Python require a NEWS entry.

Please add it using the blurb_it web app or the blurb command-line tool.

@AlexWaygood You’ve been the only human to interact with this PR so far, do you possibly have any advice on how to move this forward?

Hi @marcelm — sorry for the delay in anybody looking at this. I haven't studied your PR in detail (or thought about whether the proposal is a good idea), but it looks well put together at first glance.

I'll try to take a look soon. I won't be able to review the C code, but I can comment on whether the proposal seems like a good idea, and I can review the Python implementation and the tests.

Note that if this proposal is accepted, it will also need:

Updates to the docs for BytesIO (Doc/library/io.rst)
An entry in "What's new in Python 3.12" (Doc/whatsnew/3.12.rst)

You can also add yourself to Misc/ACKS as part of this PR, and give yourself credit in the NEWS entry, if you like. (Neither is compulsory.)

Thank you! I pushed a documentation update and will add an entry to the What’s new document in case the PR is reviewed favorably.

I'll defer to @benjaminp's judgement on this one -- I'm really not the right reviewer for this, unfortunately :(

Please ping me again if you still haven't had a review in a few weeks.

Thank you! I appreciate that you took the time.

Please ping me again if you still haven't had a review in a few weeks.

@AlexWaygood Hi! Been 8 months, no review so far :) How can we take this forward? I'm not the original author of the PR but I also have a use-case for this API and would like to see this change land, happy to help progress this.

Thanks for your interest! I have updated the PR to fix the merge conflict and to reflect that it now needs to target 3.13.

@marcelm That was quick, thank you so much!

cc: @erlend-aasland @vstinner @benjaminp for the io C code.

A

Wait. Why can it more than requested?

Apparently size=0 means "read all". Please document it.

This was just copied from the documentation for BufferedReader.peek(), but I agree that it could be written better. Changed now.

Wait. Why can it more than requested?

BufferedReader.peek() ignores the size argument and just returns whatever it has in its internal buffer.

I would prefer that BytesIO.peek() documents its behavior, rather than BufferedReader.peek() behavior.

Absolutely, it’s already changed.

(BTW, is there a policy on who clicks the "Resolve conversation" button?)

Would it be possible to implement it without touching the position? This code is not thread safe. I don't know if it's supposed to be thread-safe. Maybe add a private read method which has an argument to decide to move the position or not.

Same remark for C code.

Good point; I implemented this now so that the position is not changed. I factored out a peek_bytes function in the C version that does not advance the position.

It seems though that neither the C nor the Python version of BytesIO are supposed to be thread safe. (They don’t use locks as e.g. BufferedReader does.) So I would suggest making them so would be a task for a different PR.

All commit authors signed the Contributor License Agreement.

Commits 533
Files changed 1299

Oh oh. Something went wrong with this PR :-( I suggest to abandon this PR and create a new one.

Sorry, I merged main manually; it seems this triggered a lot of stuff I did not intend to.

I will open a new PR.

The new PR: #150917.

@marcelm, probably you forgot to pull changes from upstream to your main branch or to your feature branch.

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marcelm force-pushed the fix-issue-46375 branch from 3111318 to 675718d Compare April 11, 2022 20:06

marcelm changed the title bpo-46375: Implement BytesIO.peek() gh-90533: Implement BytesIO.peek() Apr 11, 2022

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AlexWaygood added the topic-IO label Apr 14, 2022

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marcelm force-pushed the fix-issue-46375 branch from 675718d to d4e849f Compare August 5, 2022 22:44

marcelm force-pushed the fix-issue-46375 branch from ef5eb69 to b82fa85 Compare August 5, 2022 23:28

marcelm force-pushed the fix-issue-46375 branch 2 times, most recently from 6998be1 to be39ff2 Compare November 6, 2022 15:11

AlexWaygood added the type-feature A feature request or enhancement label Nov 9, 2022

AlexWaygood self-requested a review November 9, 2022 13:37

kumaraditya303 requested a review from benjaminp November 27, 2022 07:18

AlexWaygood removed their request for review November 28, 2022 12:26

marcelm force-pushed the fix-issue-46375 branch from 5cb3143 to 79d5032 Compare September 19, 2023 08:04

vstinner reviewed Sep 21, 2023

View reviewed changes

Do not cap at DEFAULT_BUFFER_SIZE

32fd791

marcelm mentioned this pull request Jun 4, 2026

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

Merged

+                      pos = self.tell()
+                      if size == 0:
+                          size = -1
+                      b = self.read(size)

+                    .. versionadded:: 3.13
+                    Return bytes from the current position onwards but without advancing the
+                    position. The number of bytes returned may be less or more than requested.

gh-90533: Implement BytesIO.peek() by marcelm · Pull Request #30808 · python/cpython · GitHub

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gh-90533: Implement BytesIO.peek() - #30808

gh-90533: Implement BytesIO.peek()#30808marcelm wants to merge 41 commits intopython:mainpython/cpython:mainfrom marcelm:fix-issue-46375marcelm/cpython:fix-issue-46375Copy head branch name to clipboard

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gh-90533: Implement BytesIO.peek()#30808
marcelm wants to merge 41 commits into
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marcelm:fix-issue-46375

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