gh-149046: fix: correctly handle str subclasses in StringIO

[KowalskiThomas]

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.

importioclassStrSubclass(str):
def__str__(self):
return"WRONG_VALUE"obj=StrSubclass("correct_value")
buf=io.StringIO()
buf.write(obj)
value=buf.getvalue()
assertvalue=="correct_value", f"got {value!r}"

... gives AssertionError: got 'WRONG_VALUE'

• Issue: StringIO breaking change for str subclasses #149046

[serhiy-storchaka]

LGTM. 👍

@vstinner, should not we add PyUnicodeWriter_WriteRawStr()?

[vstinner]

Oh, that's the same issue that gh-148241 which was fixed in the JSON, also using the private function:

// gh-148241: Avoid PyUnicodeWriter_WriteStr() which calls str(obj)// on str subclassesif (_PyUnicodeWriter_WriteStr((_PyUnicodeWriter*)writer, pystr) <0) {
return-1;
}

@vstinner, should not we add PyUnicodeWriter_WriteRawStr()?

Yes, we should add a new function since the issue was identified in at least two places of the Python stdlib. So there is a need for such API.

[vstinner]

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).

[miss-islington-app]

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

[bedevere-app]

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

[bedevere-app]

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

[vstinner]

Merged, thanks for the fix @KowalskiThomas.

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

[serhiy-storchaka]

Go ahead!