PEP 528 – Change Windows console encoding to UTF-8
- Steve Dower <steve.dower at python.org>
- Standards Track
- 01-Sep-2016, 04-Sep-2016
- Python-Dev message
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.
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
_io.WindowsConsoleIO that acts as a raw IO object using the Windows
console functions, specifically,
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). Code that assumes
a particular encoding for the standard streams other than ASCII will likely
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.
PyRun_InteractiveOneObject which currently obtains the encoding
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
will enable the legacy-mode flag, which completely restores the previous
The win_unicode_console package 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
'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' >>> >>>
>>> is due to the newline remaining in the input buffer and is
expected in this situation.)
This document has been placed in the public domain.
Last modified: 2017-11-11 19:28:55 GMT