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PEP 528 -- Change Windows console encoding to UTF-8

PEP: 528
Title: Change Windows console encoding to UTF-8
Author: Steve Dower <steve.dower at python.org>
Status: Final
Type: Standards Track
Created: 27-Aug-2016
Python-Version: 3.6
Post-History: 01-Sep-2016, 04-Sep-2016
Resolution: https://mail.python.org/pipermail/python-dev/2016-September/146278.html

Abstract

Historically, Python uses the ANSI APIs for interacting with the Windows operating system, often via C Runtime functions. However, these have been long discouraged in favor of the UTF-16 APIs. Within the operating system, all text is represented as UTF-16, and the ANSI APIs perform encoding and decoding using the active code page.

This PEP proposes changing the default standard stream implementation on Windows to use the Unicode APIs. This will allow users to print and input the full range of Unicode characters at the default Windows console. This also requires a subtle change to how the tokenizer parses text from readline hooks.

Specific Changes

Add _io.WindowsConsoleIO

Currently an instance of _io.FileIO is used to wrap the file descriptors representing standard input, output and error. We add a new class (implemented in C) _io.WindowsConsoleIO that acts as a raw IO object using the Windows console functions, specifically, ReadConsoleW and WriteConsoleW .

This class will be used when the legacy-mode flag is not in effect, when opening a standard stream by file descriptor and the stream is a console buffer rather than a redirected file. Otherwise, _io.FileIO will be used as it is today.

This is a raw (bytes) IO class that requires text to be passed encoded with utf-8, which will be decoded to utf-16-le and passed to the Windows APIs. Similarly, bytes read from the class will be provided by the operating system as utf-16-le and converted into utf-8 when returned to Python.

The use of an ASCII compatible encoding is required to maintain compatibility with code that bypasses the TextIOWrapper and directly writes ASCII bytes to the standard streams (for example, Twisted's process_stdinreader.py [1] ). Code that assumes a particular encoding for the standard streams other than ASCII will likely break.

Add _PyOS_WindowsConsoleReadline

To allow Unicode entry at the interactive prompt, a new readline hook is required. The existing PyOS_StdioReadline function will delegate to the new _PyOS_WindowsConsoleReadline function when reading from a file descriptor that is a console buffer and the legacy-mode flag is not in effect (the logic should be identical to above).

Since the readline interface is required to return an 8-bit encoded string with no embedded nulls, the _PyOS_WindowsConsoleReadline function transcodes from utf-16-le as read from the operating system into utf-8.

The function PyRun_InteractiveOneObject which currently obtains the encoding from sys.stdin will select utf-8 unless the legacy-mode flag is in effect. This may require readline hooks to change their encodings to utf-8, or to require legacy-mode for correct behaviour.

Add legacy mode

Launching Python with the environment variable PYTHONLEGACYWINDOWSSTDIO set will enable the legacy-mode flag, which completely restores the previous behaviour.

Alternative Approaches

The win_unicode_console package [2] is a pure-Python alternative to changing the default behaviour of the console. It implements essentially the same modifications as described here using pure Python code.

Code that may break

The following code patterns may break or see different behaviour as a result of this change. All of these code samples require explicitly choosing to use a raw file object in place of a more convenient wrapper that would prevent any visible change.

Assuming stdin/stdout encoding

Code that assumes that the encoding required by sys.stdin.buffer or sys.stdout.buffer is 'mbcs' or a more specific encoding may currently be working by chance, but could encounter issues under this change. For example:

>>> sys.stdout.buffer.write(text.encode('mbcs'))
>>> r = sys.stdin.buffer.read(16).decode('cp437')

To correct this code, the encoding specified on the TextIOWrapper should be used, either implicitly or explicitly:

>>> # Fix 1: Use wrapper correctly
>>> sys.stdout.write(text)
>>> r = sys.stdin.read(16)

>>> # Fix 2: Use encoding explicitly
>>> sys.stdout.buffer.write(text.encode(sys.stdout.encoding))
>>> r = sys.stdin.buffer.read(16).decode(sys.stdin.encoding)

Incorrectly using the raw object

Code that uses the raw IO object and does not correctly handle partial reads and writes may be affected. This is particularly important for reads, where the number of characters read will never exceed one-fourth of the number of bytes allowed, as there is no feasible way to prevent input from encoding as much longer utf-8 strings:

>>> raw_stdin = sys.stdin.buffer.raw
>>> data = raw_stdin.read(15)
abcdefghijklm
b'abc'
# data contains at most 3 characters, and never more than 12 bytes
# error, as "defghijklm\r\n" is passed to the interactive prompt

To correct this code, the buffered reader/writer should be used, or the caller should continue reading until its buffer is full:

>>> # Fix 1: Use the buffered reader/writer
>>> stdin = sys.stdin.buffer
>>> data = stdin.read(15)
abcedfghijklm
b'abcdefghijklm\r\n'

>>> # Fix 2: Loop until enough bytes have been read
>>> raw_stdin = sys.stdin.buffer.raw
>>> b = b''
>>> while len(b) < 15:
...     b += raw_stdin.read(15)
abcedfghijklm
b'abcdefghijklm\r\n'

Using the raw object with small buffers

Code that uses the raw IO object and attempts to read less than four characters will now receive an error. Because it's possible that any single character may require up to four bytes when represented in utf-8, requests must fail:

>>> raw_stdin = sys.stdin.buffer.raw
>>> data = raw_stdin.read(3)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: must read at least 4 bytes

The only workaround is to pass a larger buffer:

>>> # Fix: Request at least four bytes
>>> raw_stdin = sys.stdin.buffer.raw
>>> data = raw_stdin.read(4)
a
b'a'
>>> >>>

(The extra >>> is due to the newline remaining in the input buffer and is expected in this situation.)

Source: https://github.com/python/peps/blob/master/pep-0528.txt