[3.15] gh-155742: Test PyBytesWriter_FinishWithSize() with negative size (GH-155783) (#155791)

https://github.com/python/cpython/commit/fe4a14eff16fd4574bc1037b1ef0a0e52718efeb
commit: fe4a14eff16fd4574bc1037b1ef0a0e52718efeb
branch: 3.15
author: Miss Islington (bot) <[email protected]>
committer: hugovk <[email protected]>
date: 2026-08-19T06:19:11+03:00
summary:

[3.15] gh-155742: Test PyBytesWriter_FinishWithSize() with negative size (GH-155783) (#155791)

Co-authored-by: Victor Stinner <[email protected]>

files:
M Lib/test/test_capi/test_bytes.py

diff --git a/Lib/test/test_capi/test_bytes.py b/Lib/test/test_capi/test_bytes.py
index c591e986bbce35f..d20e5016f969c2f 100644
--- a/Lib/test/test_capi/test_bytes.py
+++ b/Lib/test/test_capi/test_bytes.py
@@ -315,10 +315,16 @@ def test_create(self):
         self.assertEqual(writer.get_size(), 3)
         self.assertEqual(writer.finish(), self.result_type(b'abc'))
 
+    def test_finish_with_size(self):
+        # Test PyBytesWriter_FinishWithSize()
         writer = self.create_writer(10, b'abc')
         self.assertEqual(writer.get_size(), 10)
         self.assertEqual(writer.finish_with_size(3), self.result_type(b'abc'))
 
+        writer = self.create_writer(3, b'abc')
+        with self.assertRaises(SystemError):
+            writer.finish_with_size(-3)
+
     def test_write_bytes(self):
          # Test PyBytesWriter_WriteBytes()
          writer = self.create_writer()

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