[C API] Add Python PyUnicodeWriter API to the limited C API 3.16 #161172

Currently, the limited C API has only few functions to create a Unicode string, as such PyUnicode_FromString() (from an UTF-8 string) or PyUnicode_FromWideChar() (from a wide string). The C API optimized for PEP 393 "compact strings" (added to Python 3.3) such as PyUnicode_New() and PyUnicode_WRITE() is excluded from the limited C API.

Python 3.14 adds PyUnicodeWriter API which is an efficient way to create a new str object from various string formats (ASCII, UTF-8, wide string, UCS4 string, etc.). This API doesn't expose implementation details (how strings are stored internally), so Python implementations other than CPython should be able to implement this C API in an efficient way.

So I propose adding these 13 functions to the limited C API:

• PyUnicodeWriter_Create()
• PyUnicodeWriter_DecodeUTF8Stateful()
• PyUnicodeWriter_Discard()
• PyUnicodeWriter_Finish()
• PyUnicodeWriter_Format()
• PyUnicodeWriter_WriteASCII()
• PyUnicodeWriter_WriteChar()
• PyUnicodeWriter_WriteRepr()
• PyUnicodeWriter_WriteStr()
• PyUnicodeWriter_WriteSubstring()
• PyUnicodeWriter_WriteUCS4()
• PyUnicodeWriter_WriteUTF8()
• PyUnicodeWriter_WriteWideChar()

See the PyUnicodeWriter documentation.

In the PyPI top 15,000 projects (at 2026-09-17), I found 3 projects already using this Python 3.14 API:

• asyncpg (0.31.0)
• multidict (6.8.0)
• simplejson (4.1.2)

Adding this API to the limited C API should help more projects to adopt it.

Note: the pythoncapi-compat project also provides the PyUnicodeWriter API on Python 3.6 - Python 3.13.

This change was approved by C API Working Group.

Linked PRs

• gh-161172: Add PyUnicodeWriter to the limited C API #161175