https://github.com/python/cpython/commit/bab97c599f7c66b6b7b718a1c03a4bf4520b504c
commit: bab97c599f7c66b6b7b718a1c03a4bf4520b504c
branch: main
author: Victor Stinner <[email protected]>
committer: vstinner <[email protected]>
date: 2026-09-12T20:12:45Z
summary:
gh-157242: Allow negative grow in PyBytesWriter_Grow() (#157375)
PyBytesWriter_Resize() doesn't need to call byteswriter_resize() if
the buffer is already large enough.
files:
M Doc/c-api/bytes.rst
M Lib/test/test_capi/test_bytes.py
M Objects/bytesobject.c
diff --git a/Doc/c-api/bytes.rst b/Doc/c-api/bytes.rst
index ff68ecafcda4d0..bf89481a628ed2 100644
--- a/Doc/c-api/bytes.rst
+++ b/Doc/c-api/bytes.rst
@@ -429,7 +429,7 @@ Low-level API
On success, return ``0``.
On error, set an exception and return ``-1``.
- *size* can be negative to shrink the writer.
+ *grow* can be negative to shrink the writer.
.. c:function:: void* PyBytesWriter_GrowAndUpdatePointer(PyBytesWriter
*writer, Py_ssize_t size, void *buf)
diff --git a/Lib/test/test_capi/test_bytes.py b/Lib/test/test_capi/test_bytes.py
index 1500faa7f71344..6c19ad14b7e6c5 100644
--- a/Lib/test/test_capi/test_bytes.py
+++ b/Lib/test/test_capi/test_bytes.py
@@ -461,6 +461,24 @@ def test_grow(self):
writer.grow(0) # noop
self.assertEqual(writer.finish(), b'number=123')
+ for size in (self.SMALL_BUFFER, self.LARGE_BUFFER):
+ with self.subTest(size=size):
+ # Truncate the last byte
+ data = b'x' * size
+ writer = self.create_writer(size)
+ writer.write(0, data)
+ self.assertEqual(writer.get_data(), data)
+ writer.grow(-1)
+ self.assertEqual(writer.get_data(), data[:-1])
+ self.assertEqual(writer.finish(), data[:-1])
+
+ # Make the buffer empty
+ writer = self.create_writer(size)
+ writer.write(0, data)
+ writer.grow(-size)
+ self.assertEqual(writer.get_data(), b'')
+ self.assertEqual(writer.finish(), b'')
+
# Switch from small buffer to large buffer
writer = self.create_writer()
small, large = self.SMALL_BUFFER, self.LARGE_BUFFER
@@ -476,8 +494,8 @@ def test_grow(self):
with self.subTest(size=size):
writer = self.create_writer()
writer.write_bytes(b'x' * size, -1)
- with self.assertRaisesRegex(ValueError, 'size must be >= 0'):
- writer.grow(-1)
+ with self.assertRaisesRegex(ValueError, 'invalid size'):
+ writer.grow(-size - 1)
with self.assertRaises(MemoryError):
writer.grow(_testcapi.PY_SSIZE_T_MAX)
self.assertEqual(writer.finish(), b'x' * size)
diff --git a/Objects/bytesobject.c b/Objects/bytesobject.c
index ace5fe9d86a7d3..deddb8d959b157 100644
--- a/Objects/bytesobject.c
+++ b/Objects/bytesobject.c
@@ -3845,8 +3845,13 @@ PyBytesWriter_Resize(PyBytesWriter *writer, Py_ssize_t
size)
PyErr_SetString(PyExc_ValueError, "size must be >= 0");
return -1;
}
- if (byteswriter_resize(writer, size, 1) < 0) {
- return -1;
+ if (writer->size < size) {
+ if (byteswriter_resize(writer, size, 1) < 0) {
+ return -1;
+ }
+ }
+ else {
+ // The buffer is already large enough. Never shrink the buffer.
}
writer->size = size;
return 0;
@@ -3866,22 +3871,26 @@ _PyBytesWriter_ResizeAndUpdatePointer(PyBytesWriter
*writer, Py_ssize_t size,
int
-PyBytesWriter_Grow(PyBytesWriter *writer, Py_ssize_t size)
+PyBytesWriter_Grow(PyBytesWriter *writer, Py_ssize_t grow)
{
- if (size < 0) {
- PyErr_SetString(PyExc_ValueError, "size must be >= 0");
- return -1;
- }
- if (size == 0) {
+ if (grow == 0) {
// Nothing to do
return 0;
}
- if (size > PY_SSIZE_T_MAX - writer->size) {
- PyErr_NoMemory();
- return -1;
+ if (grow >= 0) {
+ if (grow > PY_SSIZE_T_MAX - writer->size) {
+ PyErr_NoMemory();
+ return -1;
+ }
+ }
+ else {
+ if (writer->size + grow < 0) {
+ PyErr_SetString(PyExc_ValueError, "invalid size");
+ return -1;
+ }
}
- size = writer->size + size;
+ Py_ssize_t size = writer->size + grow;
if (byteswriter_resize(writer, size, 1) < 0) {
return -1;
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