gh-71810: fix corner case (length==0) for int.to_bytes() by skirpichev · Pull Request #138739 · python/cpython

skirpichev · 2025-09-10T10:55:33Z

>>> (0).to_bytes(0, 'big', signed=True)
b''
>>> (-1).to_bytes(0, 'big', signed=True)  # was b''
Traceback (most recent call last):
  File "<python-input-0>", line 1, in <module>
    (-1).to_bytes(0, 'big', signed=True)
    ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^
OverflowError: int too big to convert

Issue: int.to_bytes() and int.from_bytes(): raise ValueError when bytes count is zero #71810

```pycon >>> (0).to_bytes(0, 'big', signed=True) b'' >>> (-1).to_bytes(0, 'big', signed=True) # was b'' Traceback (most recent call last): File "<python-input-0>", line 1, in <module> (-1).to_bytes(0, 'big', signed=True) ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^ OverflowError: int too big to convert ```

vstinner

LGTM

vstinner · 2025-09-10T11:46:31Z

@serhiy-storchaka: I would suggest to backport this change to Python 3.13 and 3.14. What do you think?

serhiy-storchaka

It seems there are no enough tests for signed=True. Please add also tests for (128).to_bytes(0, 'big', signed=True) and (-129).to_bytes(0, 'big', signed=True).

skirpichev · 2025-09-10T12:04:46Z

Disclaimer:
This pr implements "the minimal change necessary to remove ambiguity" (c) @mdickinson

I was thinking about (0).to_bytes(0, 'big', signed=True) too, but that means rejecting also int.from_bytes(b"", ..., signed=True). That case looks for me as a special case, that aren't special enough to warrant compatibility break.

serhiy-storchaka · 2025-09-10T12:08:26Z

Or better add several tests for extreme values and values just above the limit for all combination of signedness and n=0, 1, 2.

skirpichev · 2025-09-10T12:24:07Z

It seems there are no enough tests for signed=True. Please add also tests for (128).to_bytes(0, 'big', signed=True) and (-129).to_bytes(0, 'big', signed=True).

Done.

There are such tests, but rather for "automatic" value for length parameter (determined by result):

cpython/Lib/test/test_long.py

Line 1262 in 1ce0553

def test_to_bytes(self):

Or better add several tests for extreme values and values just above the limit for all combination of signedness and n=0, 1, 2.

It seems to be a big refactoring of tests. Maybe it does make sense as a separate pr?

Lib/test/test_long.py

serhiy-storchaka

I would add much more tests. Anyway, LGTM. 👍

Lib/test/test_long.py

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

miss-islington-app · 2025-09-11T10:30:57Z

Thanks @skirpichev for the PR, and @vstinner for merging it 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14.
🐍🍒⛏🤖

…nGH-138739) ```pycon >>> (0).to_bytes(0, 'big', signed=True) b'' >>> (-1).to_bytes(0, 'big', signed=True) # was b'' Traceback (most recent call last): File "<python-input-0>", line 1, in <module> (-1).to_bytes(0, 'big', signed=True) ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^ OverflowError: int too big to convert ``` (cherry picked from commit 011179a) Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com> Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

bedevere-app · 2025-09-11T10:31:10Z

GH-138782 is a backport of this pull request to the 3.14 branch.

bedevere-app · 2025-09-11T10:31:16Z

GH-138783 is a backport of this pull request to the 3.13 branch.

vstinner · 2025-09-11T10:31:43Z

Merged, thank you.

…38739) (#138783) gh-71810: Fix corner case (length==0) for int.to_bytes() (GH-138739) ```pycon >>> (0).to_bytes(0, 'big', signed=True) b'' >>> (-1).to_bytes(0, 'big', signed=True) # was b'' Traceback (most recent call last): File "<python-input-0>", line 1, in <module> (-1).to_bytes(0, 'big', signed=True) ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^ OverflowError: int too big to convert ``` (cherry picked from commit 011179a) Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com> Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

…38739) (#138782) Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com> Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

vstinner · 2025-09-13T17:31:55Z

Objects/longobject.c

        p += pincr;
    }
-    else if (j == n && n > 0 && is_signed) {
+    else if (j == n && is_signed) {


This change introduced an undefined behavior: unsigned char msb = *(p - pincr); read is out of the bounds if n == 0. I wrote #138873 to fix it.

Oh, sorry. Thanks!

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int.to_bytes() and int.from_bytes(): raise ValueError when bytes count is zero #71810

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serhiy-storchaka added needs backport to 3.13 bugs and security fixes needs backport to 3.14 bugs and security fixes labels Sep 10, 2025

address review: test for 128/-129

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skirpichev and others added 2 commits September 11, 2025 10:04

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Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

+ fix typo

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vstinner merged commit 011179a into python:main Sep 11, 2025
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skirpichev deleted the int-bytes/71810 branch September 11, 2025 10:46

hugovk pushed a commit that referenced this pull request Sep 12, 2025

[3.14] gh-71810: Fix corner case (length==0) for int.to_bytes() (GH-1…

440b83f

…38739) (#138782) Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com> Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

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skirpichev commented Sep 10, 2025 •

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