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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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Merged
vstinner merged 6 commits into from
Sep 11, 2025
Conversation 21 Commits 6 Checks 47 Files changed
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gh-71810: fix corner case (length==0) for int.to_bytes() #138739

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33f60b9
gh-71810: fix corner case (length==0) for int.to_bytes()
skirpichev Sep 10, 2025
ea3d52f
add news
skirpichev Sep 10, 2025
f0090e7
address review: test for 128/-129
skirpichev Sep 10, 2025
db7182a
+1
skirpichev Sep 11, 2025
aff76b8
Apply suggestions from code review
skirpichev Sep 11, 2025
60732b7
+ fix typo
skirpichev Sep 11, 2025
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9 changes: 7 additions & 2 deletions Lib/test/test_long.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1374,17 +1374,22 @@ def equivalent_python(n, length, byteorder, signed=False):
check(tests4, 'little', signed=False)

self.assertRaises(OverflowError, (256).to_bytes, 1, 'big', signed=False)
self.assertRaises(OverflowError, (256).to_bytes, 1, 'big', signed=True)
self.assertRaises(OverflowError, (256).to_bytes, 1, 'little', signed=False)
self.assertRaises(OverflowError, (256).to_bytes, 1, 'little', signed=True)
self.assertRaises(OverflowError, (128).to_bytes, 1, 'big', signed=True)
self.assertRaises(OverflowError, (128).to_bytes, 1, 'little', signed=True)
self.assertRaises(OverflowError, (-129).to_bytes, 1, 'big', signed=True)
self.assertRaises(OverflowError, (-129).to_bytes, 1, 'little', signed=True)
self.assertRaises(OverflowError, (-1).to_bytes, 2, 'big', signed=False)
self.assertRaises(OverflowError, (-1).to_bytes, 2, 'little', signed=False)
self.assertEqual((0).to_bytes(0, 'big'), b'')
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self.assertEqual((0).to_bytes(0, 'big', signed=True), b'')
self.assertEqual((1).to_bytes(5, 'big'), b'\x00\x00\x00\x00\x01')
self.assertEqual((0).to_bytes(5, 'big'), b'\x00\x00\x00\x00\x00')
self.assertEqual((-1).to_bytes(5, 'big', signed=True),
b'\xff\xff\xff\xff\xff')
self.assertRaises(OverflowError, (1).to_bytes, 0, 'big')
self.assertRaises(OverflowError, (-1).to_bytes, 0, 'big', signed=True)
self.assertRaises(OverflowError, (-1).to_bytes, 0, 'little', signed=True)

# gh-98783
class SubStr(str):
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2025-09-10-14-53-59.gh-issue-71810.ppf0J-.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Raise :exc:`OverflowError` for ``(-1).to_bytes()`` for signed conversions
when bytes count is zero. Patch by Sergey B Kirpichev.
2 changes: 1 addition & 1 deletion Objects/longobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1209,7 +1209,7 @@ _PyLong_AsByteArray(PyLongObject* v,
*p = (unsigned char)(accum & 0xff);
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!
/* The main loop filled the byte array exactly, so the code
just above didn't get to ensure there's a sign bit, and the
loop below wouldn't add one either. Make sure a sign bit
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