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Showing content from https://github.com/python/cpython/issues/129813 below:

Add PyBytesWriter API · Issue #129813 · python/cpython · GitHub

I propose adding a PyBytesWriter API to create bytes objects.

• Efficient API thanks to overallocation in PyBytesWriter_Extend() and usage of a "small buffer" of (around) 256 bytes
• Avoid creating incomplete/inconsistent bytes objects.
• Avoid mutating immutable bytes.

API:

typedef struct PyBytesWriter PyBytesWriter;

PyAPI_FUNC(void*) PyBytesWriter_Create(
    PyBytesWriter **writer,
    Py_ssize_t alloc);
PyAPI_FUNC(void) PyBytesWriter_Discard(
    PyBytesWriter *writer);
PyAPI_FUNC(PyObject*) PyBytesWriter_Finish(
    PyBytesWriter *writer,
    void *buf);

PyAPI_FUNC(Py_ssize_t) PyBytesWriter_GetRemaining(
    PyBytesWriter *writer,
    void *buf);
PyAPI_FUNC(void*) PyBytesWriter_Extend(
    PyBytesWriter *writer,
    void *buf,
    Py_ssize_t extend);
PyAPI_FUNC(void*) PyBytesWriter_WriteBytes(
    PyBytesWriter *writer,
    void *buf,
    const void *bytes,
    Py_ssize_t size);
PyAPI_FUNC(void*) PyBytesWriter_Format(
    PyBytesWriter *writer,
    void *buf,
    const char *format,
    ...);

Simple example creating the string b"abc":

PyObject* create_abc(void)
{
    PyBytesWriter *writer;
    char *str = PyBytesWriter_Create(&writer, 3);
    if (writer == NULL) return NULL;

    memcpy(str, "abc", 3);
    str += 3;

    return PyBytesWriter_Finish(writer, str);
}

Example formatting an integer in decimal, the size is not known in advance::

PyObject* format_int(int value)
{
    PyBytesWriter *writer;
    char *str = PyBytesWriter_Create(&writer, 20);
    if (writer == NULL) return NULL;

    str += PyOS_snprintf(str, 20, "%i", value);

    return PyBytesWriter_Finish(writer, str);
}

Note: using PyBytesWriter_Format() would make this code simpler.

Example using PyBytesWriter_Extend(),smilar to bytes.center() with a different API: spaces are number of whitespaces added to the left and to the right:

static PyObject *
byteswriter_center_example(Py_ssize_t spaces, char *str, Py_ssize_t str_size)
{
    PyBytesWriter *writer;
    char *buf = PyBytesWriter_Create(&writer, spaces * 2);
    if (buf == NULL) {
        goto error;
    }
    assert(PyBytesWriter_GetRemaining(writer, buf) == spaces * 2);

    // Add left spaces
    memset(buf, ' ', spaces);
    buf += spaces;
    assert(PyBytesWriter_GetRemaining(writer, buf) == spaces);

    // Copy string
    buf = PyBytesWriter_Extend(writer, buf, str_size);
    if (buf == NULL) {
        goto error;
    }
    assert(PyBytesWriter_GetRemaining(writer, buf) == spaces + str_size);

    memcpy(buf, str, str_size);
    buf += str_size;
    assert(PyBytesWriter_GetRemaining(writer, buf) == spaces);

    // Add right spaces
    memset(buf, ' ', spaces);
    buf += spaces;
    assert(PyBytesWriter_GetRemaining(writer, buf) == 0);

    return PyBytesWriter_Finish(writer, buf);

error:
    PyBytesWriter_Discard(writer);
    return NULL;
}

No response given

My previous attempt in July/August 2024:

• Add PyBytes_Writer() API capi-workgroup/decisions#39
• gh-121710: Add PyBytesWriter API #121726

• [WIP] gh-129813: Add PyBytesWriter C API #129814
• [WIP] gh-129813: Add PyBytesWriter C API (version 2) #131520
• [WIP] gh-129813, PEP 782: Add PyBytesWriter C API #131681

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