gh-154936: Fix JSON control character error position (GH-154941)

https://github.com/python/cpython/commit/adf836ebddd89793afb6d43a79d0a0739ee5514b
commit: adf836ebddd89793afb6d43a79d0a0739ee5514b
branch: main
author: yangbaechu <[email protected]>
committer: serhiy-storchaka <[email protected]>
date: 2026-07-31T11:57:00+03:00
summary:

gh-154936: Fix JSON control character error position (GH-154941)

Report the index of the invalid literal control character instead of
the following index in the pure Python decoder.

files:
A Misc/NEWS.d/next/Library/2026-07-30-23-29-56.gh-issue-154936.a90443.rst
M Lib/json/decoder.py
M Lib/test/test_json/test_unicode.py

diff --git a/Lib/json/decoder.py b/Lib/json/decoder.py
index 364e44d40cc3073..3150a29405f75c3 100644
--- a/Lib/json/decoder.py
+++ b/Lib/json/decoder.py
@@ -97,7 +97,7 @@ def py_scanstring(s, end, strict=True,
             if strict:
                 #msg = "Invalid control character %r at" % (terminator,)
                 msg = "Invalid control character {0!r} at".format(terminator)
-                raise JSONDecodeError(msg, s, end)
+                raise JSONDecodeError(msg, s, end - 1)
             else:
                 _append(terminator)
                 continue
diff --git a/Lib/test/test_json/test_unicode.py b/Lib/test/test_json/test_unicode.py
index 1aa9546dc463061..291580cb047be46 100644
--- a/Lib/test/test_json/test_unicode.py
+++ b/Lib/test/test_json/test_unicode.py
@@ -44,7 +44,13 @@ def test_ascii_non_printable_decode(self):
                          '\b\t\n\f\r')
         s = ''.join(map(chr, range(32)))
         for c in s:
-            self.assertRaises(self.JSONDecodeError, self.loads, f'"{c}"')
+            with self.subTest(control_character=ord(c)):
+                with self.assertRaises(self.JSONDecodeError) as cm:
+                    self.loads(f'"a{c}b"')
+                error = cm.exception
+                self.assertEqual(error.pos, 2)
+                self.assertEqual(error.lineno, 1)
+                self.assertEqual(error.colno, 3)
         self.assertEqual(self.loads(f'"{s}"', strict=False), s)
         self.assertEqual(self.loads('"\x7f"'), '\x7f')
 
diff --git a/Misc/NEWS.d/next/Library/2026-07-30-23-29-56.gh-issue-154936.a90443.rst b/Misc/NEWS.d/next/Library/2026-07-30-23-29-56.gh-issue-154936.a90443.rst
new file mode 100644
index 000000000000000..19660c406eb2d0f
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-07-30-23-29-56.gh-issue-154936.a90443.rst
@@ -0,0 +1,2 @@
+Fix the pure Python :mod:`json` decoder to report the correct position for
+invalid literal control characters in JSON strings.

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