python · vstinner · Nov 6, 2025 · Nov 6, 2025
diff --git a/Lib/test/test_bytes.py b/Lib/test/test_bytes.py
@@ -769,6 +769,13 @@ def __int__(self):
            with self.assertRaisesRegex(TypeError, msg):
                operator.mod(format_bytes, value)

+    def test_memory_leak_gh_140939(self):
+        # gh-140939: MemoryError is raised without leaking
+        _testcapi = import_helper.import_module('_testcapi')
+        with self.assertRaises(MemoryError):
+            b = self.type2test(b'%*b')
+            b % (_testcapi.PY_SSIZE_T_MAX, b'abc')
+
    def test_imod(self):
        b = self.type2test(b'hello, %b!')
        orig = b

diff --git a/Misc/NEWS.d/next/Core and Builtins/2025-11-03-17-21-38.gh-issue-140939.FVboAw.rst b/Misc/NEWS.d/next/Core and Builtins/2025-11-03-17-21-38.gh-issue-140939.FVboAw.rst
@@ -0,0 +1,2 @@
+Fix memory leak when :class:`bytearray` or :class:`bytes` is formated with the
+``%*b`` format with a large width that results in a :exc:`MemoryError`.
diff --git a/Objects/bytesobject.c b/Objects/bytesobject.c
@@ -965,8 +965,10 @@ _PyBytes_FormatEx(const char *format, Py_ssize_t format_len,
            /* 2: size preallocated for %s */
            if (alloc > 2) {
                res = _PyBytesWriter_Prepare(&writer, res, alloc - 2);
-                if (res == NULL)
+                if (res == NULL) {
+                    Py_XDECREF(temp);
                    goto error;
+                }
            }
 #ifndef NDEBUG
            char *before = res;
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		Fix memory leak when :class:`bytearray` or :class:`bytes` is formated with the
		``%*b`` format with a large width that results in a :exc:`MemoryError`.