gh-151814: Fix unbounded memory growth from repeated empty writes to io.TextIOWrapper

[StanFromIreland]

• Issue: Runaway memory use when repeatedly printing empty string #151814

[StanFromIreland]

Git seems to render the diff poorly, locally I see with -w (--ignore-all-space):

$ git show -w HEAD -- Modules/_io/textio.c
commit c6b5163133619febd0fbe8c327e52399b1a54ffd (HEAD -> textio-acc, origin/textio-acc)
Author: Stan Ulbrych <stan@python.org>
Date: Sat Jun 20 21:16:54 2026 +0100
Fix unbounded memory growth from repeated empty writes to io.TextIOWrapper
diff --git a/Modules/_io/textio.c b/Modules/_io/textio.c
index 24e08cec88f..5b2a20a30c2 100644
--- a/Modules/_io/textio.c+++ b/Modules/_io/textio.c@@ -1820,6 +1820,7 @@ _io_TextIOWrapper_write_impl(textio *self, PyObject *text)
}
}
+ if (bytes_len > 0) {
if (self->pending_bytes == NULL) {
assert(self->pending_bytes_count == 0);
self->pending_bytes = b;
@@ -1846,6 +1847,11 @@ _io_TextIOWrapper_write_impl(textio *self, PyObject *text)
}
self->pending_bytes_count += bytes_len;
+ }+ else {+ Py_DECREF(b);+ }+
if (self->pending_bytes_count >= self->chunk_size || needflush ||
text_needflush) {
if (_textiowrapper_writeflush(self) < 0)

[asottile]

you can use ?w=1 on GitHub fwiw

[StanFromIreland]

Cool, thanks! But, it seems to be limited and unfortunately doesn’t support all options, ?ignore-all-space=1 didn't work for me.

[miss-islington-app]

Thanks @StanFromIreland for the PR 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14, 3.15.
🐍🍒⛏🤖

[StanFromIreland]

Thanks for the review!

[bedevere-app]

GH-152071 is a backport of this pull request to the 3.15 branch.

[bedevere-app]

GH-152072 is a backport of this pull request to the 3.14 branch.

[bedevere-app]

GH-152073 is a backport of this pull request to the 3.13 branch.