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Original file line number	Diff line number	Diff line change
Expand Up		@@ -75,6 +75,8 @@ PyAPI_FUNC(PyObject ) _PyBytes_Repeat(PyObject self, Py_ssize_t n);
		*/
		#define _PyBytesObject_SIZE (offsetof(PyBytesObject, ob_sval) + 1)

		extern int _PyBytes_ResizeKeepOnError(PyObject **pv, Py_ssize_t newsize);

		/* --- PyBytesWriter ------------------------------------------------------ */

		struct PyBytesWriter {
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59 changes: 59 additions & 0 deletions 59 Lib/test/test_bytes.py

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Original file line number	Diff line number	Diff line change
Expand Up		@@ -1555,6 +1555,46 @@ def test_resize(self):
		self.assertRaises(MemoryError, bytearray().resize, sys.maxsize)
		self.assertRaises(MemoryError, bytearray(1000).resize, sys.maxsize)

		def test_resize_error(self):
		# gh-157242: If bytearray.resize() fails (memory allocation failure),
		# the bytearray must be left unchanged.
		_testcapi = import_helper.import_module('_testcapi')

		# Simple bytearray
		data = b'some data'
		ba = bytearray(data)
		with self.assertRaises(MemoryError):
		try:
		_testcapi.set_nomemory(0)
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		ba.resize(1024)
		finally:
		_testcapi.remove_mem_hooks()
		self.assertEqual(ba, bytearray(data))

		# growing bytearray with non-zero logical start
		ba = bytearray(b'0123456789')
		expected = ba[3:]
		del ba[:3]
		with self.assertRaises(MemoryError):
		try:
		_testcapi.set_nomemory(0)
		ba.resize(1024)
		finally:
		_testcapi.remove_mem_hooks()
		self.assertEqual(ba, expected)

		# shrink bytearray with non-zero logical start
		ba = bytearray(b'0123456789')
		expected = ba[3:]
		del ba[:3]
		with self.assertRaises(MemoryError):
		try:
		_testcapi.set_nomemory(0)
		ba.resize(1)
		finally:
		_testcapi.remove_mem_hooks()
		self.assertEqual(ba, expected)

		def test_take_bytes(self):
		ba = bytearray(b'ab')
		self.assertEqual(ba.take_bytes(), b'ab')
Expand Down Expand Up		@@ -1619,6 +1659,25 @@ def test_take_bytes(self):
		self.assertEqual(ba, bytearray(b'A'))
		self.assertEqual(ord(b'c'), ord('c'))

		def test_take_bytes_error(self):
		# gh-157242: If bytearray.take_bytes() fails (memory allocation
		# failure), the bytearray must be left unchanged.
		_testcapi = import_helper.import_module('_testcapi')

		for mem_error in (0, 1):
		for to_take in (5, None):
		with self.subTest(mem_error=mem_error, to_take=to_take):
		ba = bytearray(b'0123456789')
		expected = ba[3:]
		del ba[:3]
		with self.assertRaises(MemoryError):
		try:
		_testcapi.set_nomemory(mem_error)
		ba.take_bytes(5)
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		finally:
		_testcapi.remove_mem_hooks()
		self.assertEqual(ba, expected)

		@support.cpython_only # tests an implementation detail
		def test_take_bytes_optimization(self):
		# Validate optimization around taking lots of little chunks out of a
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15 changes: 15 additions & 0 deletions 15 Lib/test/test_capi/test_bytes.py

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Original file line number	Diff line number	Diff line change
Expand Up		@@ -389,6 +389,21 @@ def test_resize(self):
		writer.resize(len(b'number=123456'), b'456')
		self.assertEqual(writer.finish(), self.result_type(b'number=123456'))

		def test_resize_error(self):
		small_buffer = _testcapi.PyBytesWriter_small_buffer
		init = b'x' * (small_buffer * 2)
		writer = self.create_writer(len(init), init)
		size = len(init) + 100
		try:
		with self.assertRaises(MemoryError):
		_testcapi.set_nomemory(0)
		writer.resize(size, b'')
		finally:
		_testcapi.remove_mem_hooks()
		suffix = b'still working'
		writer.write_bytes(suffix, -1)
		self.assertEqual(writer.finish(), self.result_type(init + suffix))

		def test_format_i(self):
		# Test PyBytesWriter_Format()
		writer = self.create_writer()
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3 changes: 3 additions & 0 deletions 3 Misc/NEWS.d/next/C_API/2026-09-10-02-51-26.gh-issue-157242.tfXIsf.rst

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Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,3 @@
		Fix :c:func:`PyBytesWriter_Resize` to handle properly memory allocation
		failure. Leave the writer unchanged on error, instead of leaving it in an
		inconsistent state. Patch by Victor Stinner.

3 changes: 3 additions & 0 deletions 3 Misc/NEWS.d/next/Core_and_Builtins/2026-09-10-02-50-14.gh-issue-157242.LsqOUJ.rst

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Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,3 @@
		If :meth:`bytearray.resize` or :meth:`bytearray.take_bytes` fails, leave the
		:class:`bytearray` unchanged, instead of clearing it. Patch by Victor
		Stinner.

76 changes: 50 additions & 26 deletions 76 Objects/bytearrayobject.c

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Original file line number	Diff line number	Diff line change
Expand Up		@@ -281,28 +281,39 @@ bytearray_resize_lock_held(PyObject *self, Py_ssize_t requested_size)
		return -1;
		}

		/* Re-align data to the start of the allocation. */
		if (logical_offset > 0) {
		/* optimization tradeoff: This is faster than a new allocation when
		the number of bytes being removed in a resize is small; for large
		size changes it may be better to just make a new bytes object as
		_PyBytes_Resize will do a malloc + memcpy internally. */
		memmove(obj->ob_bytes, obj->ob_start,
		Py_MIN(requested_size, Py_SIZE(self)));
		}
		if (logical_offset == 0 \|\| requested_size >= Py_SIZE(self)) {
		/* Re-align data to the start of the allocation. */
		if (logical_offset > 0) {
		/* optimization tradeoff: This is faster than a new allocation when
		the number of bytes being removed in a resize is small; for large
		size changes it may be better to just make a new bytes object as
		_PyBytes_Resize will do a malloc + memcpy internally. */
		memmove(obj->ob_bytes, obj->ob_start, Py_SIZE(self));
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		obj->ob_start = obj->ob_bytes;
		}

		int ret = _PyBytes_Resize(&obj->ob_bytes_object, alloc);
		if (ret == -1) {
		obj->ob_bytes_object = Py_GetConstant(Py_CONSTANT_EMPTY_BYTES);
		size = alloc = 0;
		if (_PyBytes_ResizeKeepOnError(&obj->ob_bytes_object, alloc) < 0) {
		return -1;
		}
		}
		else {
		// Using memmove() would be unsafe, since _PyBytes_ResizeKeepOnError()
		// failure code path would be unable to restore the bytearray to its
		// previous state.
		PyObject *resized = PyBytes_FromStringAndSize(NULL, requested_size);
		if (resized == NULL) {
		return -1;
		}
		memcpy(PyBytes_AS_STRING(resized), obj->ob_start, requested_size);
		Py_SETREF(obj->ob_bytes_object, resized);
		}

		bytearray_reinit_from_bytes(obj, size, alloc);
		if (alloc != size) {
		/* Add mid-buffer null; end provided by bytes. */
		obj->ob_bytes[size] = '\0';
		}

		return ret;
		return 0;
		}

		int
Expand Down Expand Up		@@ -1610,6 +1621,7 @@ bytearray_take_bytes_impl(PyByteArrayObject self, PyObject n)
		}

		Py_ssize_t remaining_length = size - to_take;

		// optimization: If taking less than leaving, just copy the small to_take
		// portion out and move ob_start.
		if (to_take < remaining_length) {
Expand All		@@ -1631,18 +1643,30 @@ bytearray_take_bytes_impl(PyByteArrayObject self, PyObject n)
		memcpy(PyBytes_AS_STRING(remaining), self->ob_start + to_take,
		remaining_length);

		// If the bytes are offset inside the buffer must first align.
		if (self->ob_start != self->ob_bytes) {
		memmove(self->ob_bytes, self->ob_start, to_take);
		self->ob_start = self->ob_bytes;
		}
		size_t logical_offset = (size_t) (self->ob_start - self->ob_bytes);
		if (logical_offset == 0 \|\| remaining_length == 0) {
		// If the bytes are offset inside the buffer must first align.
		if (logical_offset != 0) {
		memmove(self->ob_bytes, self->ob_start, to_take);
		self->ob_start = self->ob_bytes;
		}

		if (_PyBytes_Resize(&self->ob_bytes_object, to_take) == -1) {
		assert(self->ob_bytes_object == NULL);
		self->ob_bytes_object = Py_GetConstant(Py_CONSTANT_EMPTY_BYTES);
		bytearray_reinit_from_bytes(self, 0, 0);
		Py_DECREF(remaining);
		return NULL;
		if (_PyBytes_ResizeKeepOnError(&self->ob_bytes_object, to_take) == -1) {
		Py_DECREF(remaining);
		return NULL;
		}
		}
		else {
		// Using memmove() would be unsafe, since _PyBytes_ResizeKeepOnError()
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		// failure code path would be unable to restore the bytearray to its
		// previous state.
		PyObject *resized = PyBytes_FromStringAndSize(NULL, to_take);
		if (resized == NULL) {
		Py_DECREF(remaining);
		return NULL;
		}
		memcpy(PyBytes_AS_STRING(resized), self->ob_start, to_take);
		Py_SETREF(self->ob_bytes_object, resized);
		}

		// Point the bytearray towards the buffer with the remaining data.
Expand Down

77 changes: 50 additions & 27 deletions 77 Objects/bytesobject.c

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Original file line number	Diff line number	Diff line change
Expand Up		@@ -3344,69 +3344,91 @@ PyBytes_ConcatAndDel(PyObject *pv, PyObject w)
		does not include that), and a trailing \0 byte is stored.
		*/

		// Similar to _PyBytes_Resize(), but leaves the object unchanged on error.
		int
		_PyBytes_Resize(PyObject **pv, Py_ssize_t newsize)
		_PyBytes_ResizeKeepOnError(PyObject **pv, Py_ssize_t newsize)
		{
		PyObject *v;
		PyBytesObject *sv;
		v = *pv;
		PyObject v = pv;
		PyObject *result;

		if (!PyBytes_Check(v) \|\| newsize < 0) {
		*pv = 0;
		Py_DECREF(v);
		PyErr_BadInternalCall();
		return -1;
		}

		Py_ssize_t oldsize = PyBytes_GET_SIZE(v);
		if (oldsize == newsize) {
		/* return early if newsize equals to v->ob_size */
		return 0;
		}

		if (oldsize == 0) {
		*pv = _PyBytes_FromSize(newsize, 0);
		result = _PyBytes_FromSize(newsize, 0);
		if (result == NULL) {
		return -1;
		}
		*pv = result;
		Py_DECREF(v);
		return (*pv == NULL) ? -1 : 0;
		return 0;
		}

		if (newsize == 0) {
		*pv = bytes_get_empty();
		*pv = bytes_get_empty(); // cannot fail
		Py_DECREF(v);
		return 0;
		}

		if (!_PyObject_IsUniquelyReferenced(v)) {
		// Allocate and then copy so we don't get a shared immortal
		// one-character singleton!
		*pv = _PyBytes_FromSize(newsize, 0);
		if (*pv) {
		memcpy(PyBytes_AS_STRING(*pv), PyBytes_AS_STRING(v),
		Py_MIN(oldsize, newsize));
		result = _PyBytes_FromSize(newsize, 0);
		if (!result) {
		return -1;
		}

		memcpy(PyBytes_AS_STRING(result), PyBytes_AS_STRING(v),
		Py_MIN(oldsize, newsize));
		*pv = result;
		Py_DECREF(v);
		return (*pv == NULL) ? -1 : 0;
		return 0;
		}

		assert(v != bytes_get_empty());
		result = (PyObject *)PyObject_Realloc(v, PyBytesObject_SIZE + newsize);
		if (result == NULL) {
		PyErr_NoMemory();
		return -1;
		}

		#ifdef Py_TRACE_REFS
		_Py_ForgetReference(v);
		#endif
		_PyReftracerTrack(v, PyRefTracer_DESTROY);
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		pv = (PyObject )
		PyObject_Realloc(v, PyBytesObject_SIZE + newsize);
		if (*pv == NULL) {
		#ifdef Py_REF_DEBUG
		_Py_DecRefTotal(_PyThreadState_GET());
		#endif
		PyObject_Free(v);
		PyErr_NoMemory();
		return -1;
		}
		_Py_NewReferenceNoTotal(*pv);
		sv = (PyBytesObject ) pv;

		v = result;
		_Py_NewReferenceNoTotal(v);
		PyBytesObject sv = (PyBytesObject )v;
		Py_SET_SIZE(sv, newsize);
		sv->ob_sval[newsize] = '\0';
		set_ob_shash(sv, -1); /* invalidate cached hash value */
		*pv = v;
		return 0;
		}


		int
		_PyBytes_Resize(PyObject **pv, Py_ssize_t newsize)
		{
		int res = _PyBytes_ResizeKeepOnError(pv, newsize);
		if (res < 0) {
		PyObject v = pv;
		*pv = NULL;
		Py_DECREF(v);
		}
		return res;
		}


		/********************* Bytes Iterator **************************/

		typedef struct {
Expand Down Expand Up		@@ -3644,7 +3666,8 @@ byteswriter_resize(PyBytesWriter *writer, Py_ssize_t size, int resize)
		}
		}
		else {
		if (_PyBytes_Resize(&writer->obj, size)) {
		if (_PyBytes_ResizeKeepOnError(&writer->obj, size)) {
		assert(writer->obj != NULL);
		return -1;
		}
		}
Expand Down

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