From: Sergey B Kirpichev <skirpichev@gmail.com>
Date: Thu, 11 Sep 2025 10:30:53 +0000 (+0300)
Subject: gh-71810: Fix corner case (length==0) for int.to_bytes() (#138739)
X-Git-Tag: v3.15.0a1~413
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=011179a79a0d7b93ce074b25b0819e96b6dd3315;p=thirdparty%2FPython%2Fcpython.git

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

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

diff --git a/Lib/test/test_long.py b/Lib/test/test_long.py
index d63bc19ed9c9..b48a8812a1a2 100644
--- a/Lib/test/test_long.py
+++ b/Lib/test/test_long.py
@@ -1374,17 +1374,22 @@ class LongTest(unittest.TestCase):
         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'')
+        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):
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-09-10-14-53-59.gh-issue-71810.ppf0J-.rst b/Misc/NEWS.d/next/Core_and_Builtins/2025-09-10-14-53-59.gh-issue-71810.ppf0J-.rst
new file mode 100644
index 000000000000..a87db44225e8
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2025-09-10-14-53-59.gh-issue-71810.ppf0J-.rst
@@ -0,0 +1,2 @@
+Raise :exc:`OverflowError` for ``(-1).to_bytes()`` for signed conversions
+when bytes count is zero.  Patch by Sergey B Kirpichev.
diff --git a/Objects/longobject.c b/Objects/longobject.c
index 287458ba2da6..63b48572ff20 100644
--- a/Objects/longobject.c
+++ b/Objects/longobject.c
@@ -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) {
         /* 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