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

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gh-71810: fix corner case (length==0) for int.to_bytes() - #138739

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vstinner merged 6 commits into
python:mainfrom
skirpichev:int-bytes/71810
Sep 11, 2025
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gh-71810: fix corner case (length==0) for int.to_bytes()#138739
vstinner merged 6 commits into
python:mainfrom
skirpichev:int-bytes/71810

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skirpichev commented Sep 10, 2025 •
edited by bedevere-app Bot
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>>> (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

pythongh-71810: fix corner case (length==0) for int.to_bytes()
33f60b9 
```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
```
bedevere-app Bot mentioned this pull request Sep 10, 2025
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skirpichev requested a review from vstinner September 10, 2025 10:56
skirpichev marked this pull request as ready for review September 10, 2025 11:22
vstinner approved these changes Sep 10, 2025
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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

LGTM

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vstinner commented Sep 10, 2025

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

skirpichev requested a review from vstinner September 10, 2025 11:55
serhiy-storchaka reviewed Sep 10, 2025
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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

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).

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

skirpichev commented Sep 10, 2025

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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 commented Sep 10, 2025

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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 commented Sep 10, 2025

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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?

serhiy-storchaka reviewed Sep 10, 2025
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Comment thread Lib/test/test_long.py Outdated
Comment thread Lib/test/test_long.py Outdated
serhiy-storchaka approved these changes Sep 11, 2025
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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

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

Comment thread Lib/test/test_long.py Outdated
Comment thread Lib/test/test_long.py Outdated
Comment thread Lib/test/test_long.py
skirpichev and others added 2 commits September 11, 2025 10:04
Apply suggestions from code review
aff76b8 
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
+ fix typo
60732b7
vstinner merged commit 011179a into python:main Sep 11, 2025
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miss-islington-app Bot commented Sep 11, 2025

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

miss-islington pushed a commit to miss-islington/cpython that referenced this pull request Sep 11, 2025
pythongh-71810: Fix corner case (length==0) for int.to_bytes() (pytho…
08fc362 
…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>
miss-islington pushed a commit to miss-islington/cpython that referenced this pull request Sep 11, 2025
pythongh-71810: Fix corner case (length==0) for int.to_bytes() (pytho…
8681aea 
…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 Bot commented Sep 11, 2025

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

bedevere-app Bot removed the needs backport to 3.14 bugs and security fixes label Sep 11, 2025

bedevere-app Bot commented Sep 11, 2025

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

bedevere-app Bot removed the needs backport to 3.13 only security fixes label Sep 11, 2025

vstinner commented Sep 11, 2025

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Merged, thank you.

skirpichev deleted the int-bytes/71810 branch September 11, 2025 10:46
vstinner pushed a commit that referenced this pull request Sep 11, 2025
[3.13] gh-71810: Fix corner case (length==0) for int.to_bytes() (GH-1…
7195d7f 
…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>
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>
vstinner reviewed Sep 13, 2025
View reviewed changes
Comment thread Objects/longobject.c
p += pincr;
}
else if (j == n && n > 0 && is_signed) {
else if (j == n && is_signed) {

vstinner Sep 13, 2025

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

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.

skirpichev Sep 14, 2025

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

Oh, sorry. Thanks!

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