https://github.com/python/cpython/commit/4e00e2504fdcf06b5d55a773b31f49b1d3d6d35c
commit: 4e00e2504fdcf06b5d55a773b31f49b1d3d6d35c
branch: main
author: Victor Stinner <[email protected]>
committer: vstinner <[email protected]>
date: 2025-09-16T00:15:32+02:00
summary:
gh-129813, PEP 782: Add PyBytesWriter.overallocate (#138941)
Disable overallocation in _PyBytes_FormatEx() at the last write.
files:
M Include/internal/pycore_bytesobject.h
M Objects/bytesobject.c
diff --git a/Include/internal/pycore_bytesobject.h
b/Include/internal/pycore_bytesobject.h
index 9f519d3ca95e92..99dd6bf1b912dd 100644
--- a/Include/internal/pycore_bytesobject.h
+++ b/Include/internal/pycore_bytesobject.h
@@ -147,6 +147,15 @@ PyAPI_FUNC(void*) _PyBytesWriter_WriteBytes(_PyBytesWriter
*writer,
PyAPI_FUNC(PyBytesWriter*) _PyBytesWriter_CreateByteArray(
Py_ssize_t size);
+
+struct PyBytesWriter {
+ char small_buffer[256];
+ PyObject *obj;
+ Py_ssize_t size;
+ int use_bytearray;
+ int overallocate;
+};
+
#ifdef __cplusplus
}
#endif
diff --git a/Objects/bytesobject.c b/Objects/bytesobject.c
index ee10f13b7bb04c..9bb1f84190a93f 100644
--- a/Objects/bytesobject.c
+++ b/Objects/bytesobject.c
@@ -836,6 +836,11 @@ _PyBytes_FormatEx(const char *format, Py_ssize_t
format_len,
if (v == NULL)
goto error;
+ if (fmtcnt == 0) {
+ /* last write: disable writer overallocation */
+ writer->overallocate = 0;
+ }
+
sign = 0;
fill = ' ';
switch (c) {
@@ -1056,6 +1061,10 @@ _PyBytes_FormatEx(const char *format, Py_ssize_t
format_len,
assert((res - before) == alloc);
#endif
} /* '%' */
+
+ /* If overallocation was disabled, ensure that it was the last
+ write. Otherwise, we missed an optimization */
+ assert(writer->overallocate || fmtcnt == 0 || use_bytearray);
} /* until end */
if (argidx < arglen && !dict) {
@@ -3746,14 +3755,6 @@ _PyBytes_Repeat(char* dest, Py_ssize_t len_dest,
// --- PyBytesWriter API -----------------------------------------------------
-struct PyBytesWriter {
- char small_buffer[256];
- PyObject *obj;
- Py_ssize_t size;
- int use_bytearray;
-};
-
-
static inline char*
byteswriter_data(PyBytesWriter *writer)
{
@@ -3802,7 +3803,8 @@ byteswriter_resize(PyBytesWriter *writer, Py_ssize_t
size, int overallocate)
return 0;
}
- if (overallocate && !writer->use_bytearray) {
+ overallocate &= writer->overallocate;
+ if (overallocate) {
if (size <= (PY_SSIZE_T_MAX - size / OVERALLOCATE_FACTOR)) {
size += size / OVERALLOCATE_FACTOR;
}
@@ -3867,6 +3869,7 @@ byteswriter_create(Py_ssize_t size, int use_bytearray)
writer->obj = NULL;
writer->size = 0;
writer->use_bytearray = use_bytearray;
+ writer->overallocate = !use_bytearray;
if (size >= 1) {
if (byteswriter_resize(writer, size, 0) < 0) {
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