PEP: 624
Title: Remove Py_UNICODE encoder APIs
Author: Inada Naoki <songofacandy@gmail.com>
Status: Final
Type: Standards Track
Created: 06-Jul-2020
Python-Version: 3.11
Post-History: 08-Jul-2020


Abstract
========

This PEP proposes to remove deprecated ``Py_UNICODE`` encoder APIs in Python 3.11:

* ``PyUnicode_Encode()``
* ``PyUnicode_EncodeASCII()``
* ``PyUnicode_EncodeLatin1()``
* ``PyUnicode_EncodeUTF7()``
* ``PyUnicode_EncodeUTF8()``
* ``PyUnicode_EncodeUTF16()``
* ``PyUnicode_EncodeUTF32()``
* ``PyUnicode_EncodeUnicodeEscape()``
* ``PyUnicode_EncodeRawUnicodeEscape()``
* ``PyUnicode_EncodeCharmap()``
* ``PyUnicode_TranslateCharmap()``
* ``PyUnicode_EncodeDecimal()``
* ``PyUnicode_TransformDecimalToASCII()``

.. note::

   :pep:`623` propose to remove
   Unicode object APIs relating to ``Py_UNICODE``. On the other hand, this PEP
   is not relating to Unicode object. These PEPs are split because they have
   different motivations and need different discussions.


Motivation
==========

In general, reducing the number of APIs that have been deprecated for
a long time and have few users is a good idea for not only it
improves the maintainability of CPython, but it also helps API users
and other Python implementations.


Rationale
=========

Deprecated since Python 3.3
---------------------------

``Py_UNICODE`` and APIs using it has been deprecated since Python 3.3.


Inefficient
-----------

All of these APIs are implemented using ``PyUnicode_FromWideChar``.
So these APIs are inefficient when user want to encode Unicode
object.


Not used widely
---------------

When searching from the top 4000 PyPI packages [1]_, only pyodbc use
these APIs.

* ``PyUnicode_EncodeUTF8()``
* ``PyUnicode_EncodeUTF16()``

pyodbc uses these APIs to encode Unicode object into bytes object.
So it is easy to fix it. [2]_


Alternative APIs
================

There are alternative APIs to accept ``PyObject *unicode`` instead of
``Py_UNICODE *``. Users can migrate to them.


========================================= ==========================================
Deprecated API                            Alternative APIs
========================================= ==========================================
``PyUnicode_Encode()``                    ``PyUnicode_AsEncodedString()``
``PyUnicode_EncodeASCII()``               ``PyUnicode_AsASCIIString()`` \(1)
``PyUnicode_EncodeLatin1()``              ``PyUnicode_AsLatin1String()`` \(1)
``PyUnicode_EncodeUTF7()``                \(2)
``PyUnicode_EncodeUTF8()``                ``PyUnicode_AsUTF8String()`` \(1)
``PyUnicode_EncodeUTF16()``               ``PyUnicode_AsUTF16String()`` \(3)
``PyUnicode_EncodeUTF32()``               ``PyUnicode_AsUTF32String()`` \(3)
``PyUnicode_EncodeUnicodeEscape()``       ``PyUnicode_AsUnicodeEscapeString()``
``PyUnicode_EncodeRawUnicodeEscape()``    ``PyUnicode_AsRawUnicodeEscapeString()``
``PyUnicode_EncodeCharmap()``             ``PyUnicode_AsCharmapString()`` \(1)
``PyUnicode_TranslateCharmap()``          ``PyUnicode_Translate()``
``PyUnicode_EncodeDecimal()``              \(4)
``PyUnicode_TransformDecimalToASCII()``    \(4)
========================================= ==========================================

Notes:

(1)
   ``const char *errors`` parameter is missing.

(2)
   There is no public alternative API. But user can use generic
   ``PyUnicode_AsEncodedString()`` instead.

(3)
   ``const char *errors, int byteorder`` parameters are missing.

(4)
   There is no direct replacement. But ``Py_UNICODE_TODECIMAL``
   can be used instead. CPython uses
   ``_PyUnicode_TransformDecimalAndSpaceToASCII`` for converting
   from Unicode to numbers instead.


Plan
====

Remove these APIs in Python 3.11. They have been deprecated already.

* ``PyUnicode_Encode()``
* ``PyUnicode_EncodeASCII()``
* ``PyUnicode_EncodeLatin1()``
* ``PyUnicode_EncodeUTF7()``
* ``PyUnicode_EncodeUTF8()``
* ``PyUnicode_EncodeUTF16()``
* ``PyUnicode_EncodeUTF32()``
* ``PyUnicode_EncodeUnicodeEscape()``
* ``PyUnicode_EncodeRawUnicodeEscape()``
* ``PyUnicode_EncodeCharmap()``
* ``PyUnicode_TranslateCharmap()``
* ``PyUnicode_EncodeDecimal()``
* ``PyUnicode_TransformDecimalToASCII()``


Alternative Ideas
=================

Replace ``Py_UNICODE*`` with ``PyObject*``
------------------------------------------

As described in the "Alternative APIs" section, some APIs don't have
public alternative APIs accepting ``PyObject *unicode`` input.
And some public alternative APIs have restrictions like missing
``errors`` and ``byteorder`` parameters.

Instead of removing deprecated APIs, we can reuse their names for
alternative public APIs.

Since we have private alternative APIs already, it is just renaming
from private name to public and deprecated names.

============================= ================================
 Rename to                     Rename from
============================= ================================
``PyUnicode_EncodeASCII()``    ``_PyUnicode_AsASCIIString()``
``PyUnicode_EncodeLatin1()``   ``_PyUnicode_AsLatin1String()``
``PyUnicode_EncodeUTF7()``     ``_PyUnicode_EncodeUTF7()``
``PyUnicode_EncodeUTF8()``     ``_PyUnicode_AsUTF8String()``
``PyUnicode_EncodeUTF16()``    ``_PyUnicode_EncodeUTF16()``
``PyUnicode_EncodeUTF32()``    ``_PyUnicode_EncodeUTF32()``
============================= ================================

Pros:

* We have a more consistent API set.

Cons:

* Backward incompatible.
* We have more public APIs to maintain for rare use cases.
* Existing public APIs are enough for most use cases, and
  ``PyUnicode_AsEncodedString()`` can be used in other cases.


Replace ``Py_UNICODE*`` with ``Py_UCS4*``
-----------------------------------------

We can replace ``Py_UNICODE`` with ``Py_UCS4`` and undeprecate
these APIs.

UTF-8, UTF-16, UTF-32 encoders support ``Py_UCS4`` internally.
So ``PyUnicode_EncodeUTF8()``, ``PyUnicode_EncodeUTF16()``, and
``PyUnicode_EncodeUTF32()`` can avoid to create a temporary Unicode
object.


Pros:

* We can avoid creating temporary Unicode object when encoding from
  ``Py_UCS4*`` into bytes object with UTF-8, UTF-16, UTF-32 codecs.

Cons:

* Backward incompatible.
* We have more public APIs to maintain for rare use cases.
* Other Python implementations that want to support Python/C API need
  to support these APIs too.
* If we change the Unicode internal representation to UTF-8 in the
  future, we need to keep UCS-4 support only for these APIs.


Replace ``Py_UNICODE*`` with ``wchar_t*``
-----------------------------------------

We can replace ``Py_UNICODE`` with ``wchar_t``. Since ``Py_UNICODE``
is typedef of ``wchar_t`` already, this is status quo.

On platforms where ``sizeof(wchar_t) == 4``, we can avoid to create a
temporary Unicode object when encoding from ``wchar_t*`` to bytes
objects using UTF-8, UTF-16, and UTF-32 codec, like the "Replace
``Py_UNICODE*`` with ``Py_UCS4*``" idea.


Pros:

* Backward compatible.
* We can avoid creating temporary Unicode object when encode from
  ``Py_UCS4*`` into bytes object with UTF-8, UTF-16, UTF-32 codecs
  on platform where ``sizeof(wchar_t) == 4``.

Cons:

* Although Windows is the most major platform that uses ``wchar_t``
  heavily, these APIs need to create a temporary Unicode object
  always because ``sizeof(wchar_t) == 2`` on Windows.
* We have more public APIs to maintain for rare use cases.
* Other Python implementations that want to support Python/C API need
  to support these APIs too.
* If we change the Unicode internal representation to UTF-8 in the
  future, we need to keep UCS-4 support only for these APIs.


Rejected Ideas
==============

Emit runtime warning
--------------------

In addition to existing compiler warning, emitting runtime
``DeprecationWarning`` is suggested.

But these APIs doesn't release GIL for now. Emitting a warning from
such APIs is not safe. See this example.

.. code-block::

   PyObject *u = PyList_GET_ITEM(list, i);  // u is borrowed reference.
   PyObject *b = PyUnicode_EncodeUTF8(PyUnicode_AS_UNICODE(u),
           PyUnicode_GET_SIZE(u), NULL);
   // Assumes u is still living reference.
   PyObject *t = PyTuple_Pack(2, u, b);
   Py_DECREF(b);
   return t;

If we emit Python warning from ``PyUnicode_EncodeUTF8()``, warning
filters and other threads may change the ``list`` and ``u`` can be
a dangling reference after ``PyUnicode_EncodeUTF8()`` returned.


Discussions
===========

* `[python-dev] Plan to remove Py_UNICODE APis except PEP 623
  <https://mail.python.org/archives/list/python-dev@python.org/thread/S7KW2U6IGXZFBMGS6WSJB26NZIBW4OLE/#S7KW2U6IGXZFBMGS6WSJB26NZIBW4OLE>`_
* `bpo-41123: Remove Py_UNICODE APIs except PEP 623
  <https://bugs.python.org/issue41123>`_
* `[python-dev] PEP 624: Remove Py_UNICODE encoder APIs
  <https://mail.python.org/archives/list/python-dev@python.org/thread/THXVM7FZVT56B7CPEDIYKJG6VMAYIEK5/#QUGBVLQNBFVNX25AEIL77WSFOHQES6LJ>`_


Objections
----------

* Removing these APIs removes ability to use codec without temporary
  Unicode.

  * Codecs can not encode Unicode buffer directly without temporary
    Unicode object since Python 3.3. All these APIs creates temporary
    Unicode object for now. So removing them doesn't reduce any
    abilities.

* Why not remove decoder APIs too?

  * They are part of stable ABI.

  * ``PyUnicode_DecodeASCII()`` and ``PyUnicode_DecodeUTF8()`` are
    used very widely. Deprecating them is not worth enough.

  * Decoder APIs can decode from byte buffer directly, without
    creating temporary bytes object. On the other hand, encoder APIs
    can not avoid temporary Unicode object.


References
==========

.. [1] Source package list chosen from top 4000 PyPI packages.
   (https://github.com/methane/notes/blob/master/2020/wchar-cache/package_list.txt)

.. [2] pyodbc -- Don't use PyUnicode_Encode API #792
   (https://github.com/mkleehammer/pyodbc/pull/792)


Copyright
=========

This document is placed in the public domain or under the
CC0-1.0-Universal license, whichever is more permissive.
