Hi,

I propose adding the following function the Python 3.15 C API.

API: int PyUnicodeWriter_WriteASCII(PyUnicodeWriter *writer, const char *str, Py_ssize_t size)

Documentation:

Write the ASCII string str into writer.

size is the string length in bytes. If size is equal to -1, call strlen(str) to get the string length.

str must only contain ASCII characters. The behavior is undefined if str contains non-ASCII characters.

On success, return 0.
On error, set an exception, leave the writer unchanged, and return -1.

The function is up to 2.0x faster than PyUnicodeWriter_WriteUTF8().

There are 2 reasons to add this function:

• Get the same performance when updating code using the old private _PyUnicodeWriter API to use the new public PyUnicodeWriter API.
• Get the best performance for new code using the public PyUnicodeWriter API: make the API appealing in term of performance.

Background: Using the public PyUnicodeWriter C API made the json module slower.

Vote:

• @encukou
• @erlend-aasland
• @mdboom
• @serhiy-storchaka
• @vstinner
• @zooba