gh-149046: fix: correctly handle `str` subclasses in `StringIO` by KowalskiThomas · Pull Request #149047 · python/cpython · GitHub
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gh-149046: fix: correctly handle str subclasses in StringIO - #149047

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vstinner merged 4 commits into
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KowalskiThomas:kowalski/fix-correctly-handle-str-subclasses-in-stringio
Jun 1, 2026
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gh-149046: fix: correctly handle str subclasses in StringIO#149047
vstinner merged 4 commits into
python:mainfrom
KowalskiThomas:kowalski/fix-correctly-handle-str-subclasses-in-stringio

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@KowalskiThomas

@KowalskiThomas KowalskiThomas commented Apr 27, 2026 •
edited by bedevere-app Bot
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What is this PR?

This PR is, in a way, a follow-up from #148241.

This other PR fixed a breaking change in behaviour where classes inheriting str would have __str__ called on them when instead of using the "underlying" str itself. The PR fixed the problem for JSON serialisation, but seemingly missed some other call sites, including StringIO. This seems to affect Python 3.14+ (but works as expected on 3.13).

This is a reproducer.

import io

class StrSubclass(str):
    def __str__(self):
        return "WRONG_VALUE"

obj = StrSubclass("correct_value")

buf = io.StringIO()
buf.write(obj)
value = buf.getvalue()
assert value == "correct_value", f"got {value!r}"

... gives AssertionError: got 'WRONG_VALUE'

@bedevere-app bedevere-app Bot mentioned this pull request Apr 27, 2026
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@KowalskiThomas
KowalskiThomas force-pushed the kowalski/fix-correctly-handle-str-subclasses-in-stringio branch from 9451278 to 47788ec Compare April 27, 2026 09:19
@KowalskiThomas
KowalskiThomas force-pushed the kowalski/fix-correctly-handle-str-subclasses-in-stringio branch from 47788ec to 4f0649e Compare April 27, 2026 09:19
@KowalskiThomas KowalskiThomas changed the title gh-149046: fix: correctly handle str subclasses in StringIO gh-149046: fix: correctly handle str subclasses in StringIO Apr 29, 2026
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KowalskiThomas marked this pull request as ready for review April 30, 2026 15:18
serhiy-storchaka
serhiy-storchaka approved these changes May 29, 2026
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LGTM. 👍

@vstinner, should not we add PyUnicodeWriter_WriteRawStr()?

@serhiy-storchaka serhiy-storchaka added needs backport to 3.14 bugs and security fixes needs backport to 3.15 pre-release feature fixes, bugs and security fixes labels May 29, 2026
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vstinner commented Jun 1, 2026

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vstinner reviewed Jun 1, 2026
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Comment thread Modules/_io/stringio.c
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LGTM. I'm fine with backporting the change to 3.14, since it's a regression from 3.13.

I introduced the regression in 3.14 with commit 732670d (PR gh-129243).

@vstinner
vstinner enabled auto-merge (squash) June 1, 2026 12:44
@vstinner
vstinner merged commit c987736 into python:main Jun 1, 2026
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@miss-islington-app

miss-islington-app Bot commented Jun 1, 2026

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Thanks @KowalskiThomas for the PR, and @vstinner for merging it 🌮🎉.. I'm working now to backport this PR to: 3.14, 3.15.
🐍🍒⛏🤖

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bedevere-app Bot commented Jun 1, 2026

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GH-150706 is a backport of this pull request to the 3.15 branch.

@bedevere-app bedevere-app Bot removed the needs backport to 3.15 pre-release feature fixes, bugs and security fixes label Jun 1, 2026
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bedevere-app Bot commented Jun 1, 2026

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GH-150707 is a backport of this pull request to the 3.14 branch.

@bedevere-app bedevere-app Bot removed the needs backport to 3.14 bugs and security fixes label Jun 1, 2026
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vstinner commented Jun 1, 2026

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Merged, thanks for the fix @KowalskiThomas.

@serhiy-storchaka: I will work on adding PyUnicodeWriter_WriteRawStr() later, unless you want to work on that.

vstinner pushed a commit that referenced this pull request Jun 1, 2026
@miss-islington @KowalskiThomas
[3.14] gh-149046: fix: correctly handle str subclasses in `io.Strin…
a2f00d4 
…gIO` (GH-149047) (#150707)

gh-149046: fix: correctly handle `str` subclasses in `io.StringIO` (GH-149047)
(cherry picked from commit c987736)

Co-authored-by: Thomas Kowalski <thom.kowa@gmail.com>
vstinner pushed a commit that referenced this pull request Jun 1, 2026
@miss-islington @KowalskiThomas
[3.15] gh-149046: fix: correctly handle str subclasses in `io.Strin…
e60e8cf 
…gIO` (GH-149047) (#150706)

gh-149046: fix: correctly handle `str` subclasses in `io.StringIO` (GH-149047)
(cherry picked from commit c987736)

Co-authored-by: Thomas Kowalski <thom.kowa@gmail.com>
@serhiy-storchaka

serhiy-storchaka commented Jun 1, 2026

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