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gh-129813, PEP 782: Add PyBytesWriter.overallocate #138941

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Merged
merged 2 commits into from
Sep 15, 2025
+21 −9
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gh-129813, PEP 782: Add PyBytesWriter.overallocate #138941

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f4de031
gh-129813, PEP 782: Add PyBytesWriter.overallocate
vstinner Sep 15, 2025
3b7b89a
Simplify byteswriter_resize()
vstinner Sep 15, 2025
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9 changes: 9 additions & 0 deletions Include/internal/pycore_bytesobject.h
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Original file line number Diff line number Diff line change
Expand Up @@ -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
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21 changes: 12 additions & 9 deletions Objects/bytesobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -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) {
Expand Down Expand Up @@ -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) {
Expand Down Expand Up @@ -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)
{
Expand Down Expand Up @@ -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;
}
Expand Down Expand Up @@ -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) {
Expand Down
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