Support PEP 383 on Windows: mbcs support of surrogateescape error handler

[vstinner]

BPO	9821
Nosy	@malemburg, @loewis, @vstinner

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2010-09-10.21:36:45.337>
created_at = <Date 2010-09-10.11:04:40.374>
labels = ['interpreter-core', 'invalid', 'library', 'expert-unicode', 'OS-windows']
title = 'Support PEP 383 on Windows: mbcs support of\tsurrogateescape error handler'
updated_at = <Date 2010-09-10.21:36:45.336>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2010-09-10.21:36:45.336>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2010-09-10.21:36:45.337>
closer = 'vstinner'
components = ['Interpreter Core', 'Library (Lib)', 'Unicode', 'Windows']
creation = <Date 2010-09-10.11:04:40.374>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 9821
keywords = []
message_count = 4.0
messages = ['116001', '116006', '116011', '116044']
nosy_count = 3.0
nosy_names = ['lemburg', 'loewis', 'vstinner']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue9821'
versions = ['Python 3.2']

[vstinner]

It would be nice to support PEP-383 (surrogateescape) on Windows, but the mbcs codec doesn't support it for performance reason. The Windows functions to encode/decode MBCS don't give the index of the unencodable/undecodable character/byte. For encoding, we can try to encode character by character (but be careful of surrogate pairs) and check that the character is a Python lone surrogate character or not (character in range U+DC80..U+DCFF). For decoding, it is more complex because MBCS can be a multibyte encoding, eg. cp65001 (Microsoft variant of utf-8, see bpo-6058). So it's not possible to encode byte per byte and we should write an heuristic to guess the right number of bytes for each call to the decode function.

--

A completly different solution is to get the MBCS code page and use the Python code page codec (eg. "cp1252") instead of "mbcs" encoding, because Python cpXXXX codecs support all Python error handlers. Example (with Python 2.6):

>>> print(u"abcŁdef".encode("cp1252", "replace"))
abc?def
>>> print(u"abcŁdef".encode("cp1252", "ignore"))
abcdef
>>> print(u"abcŁdef".encode("cp1252", "backslashreplace"))
abc\u0141def

See also bpo-8611 for the problem if the Python path cannot be encoded to mbcs (work in progress, see bpo-9425).

[malemburg]

STINNER Victor wrote:
> 
> New submission from STINNER Victor <victor.stinner@haypocalc.com>:
> 
> It would be nice to support PEP 383 (surrogateescape) on Windows, but the mbcs codec doesn't support it for performance reason. The Windows functions to encode/decode MBCS don't give the index of the unencodable/undecodable character/byte. For encoding, we can try to encode character by character (but be careful of surrogate pairs) and check that the character is a Python lone surrogate character or not (character in range U+DC80..U+DCFF). For decoding, it is more complex because MBCS can be a multibyte encoding, eg. cp65001 (Microsoft variant of utf-8, see python/cpython#50308). So it's not possible to encode byte per byte and we should write an heuristic to guess the right number of bytes for each call to the decode function.
> 
> --
> 
> A completly different solution is to get the MBCS code page and use the Python code page codec (eg. "cp1252") instead of "mbcs" encoding, because Python cpXXXX codecs support all Python error handlers. Example (with Python 2.6):
> 
>>>> print(u"abcŁdef".encode("cp1252", "replace"))
> abc?def
>>>> print(u"abcŁdef".encode("cp1252", "ignore"))
> abcdef
>>>> print(u"abcŁdef".encode("cp1252", "backslashreplace"))
> abc\u0141def

That would certainly be a better approach, provided that our
cp-encodings are indeed compatible with the Windows variants
(which unfortunately tend to often use slightly different
mappings).

We could then also alias 'mbcs' to the cp-encoding (sort of
like the reverse of what we do in site.py:aliasmbcs().

[vstinner]

Oh wait. PEP-383 is a solution to store undecodable bytes in an unicode string, but for mbcs I'm trying to get the opposite: store unicode in bytes and this is not possible (at least with PEP-383).

Example with Python 3.1:

>>> print("abcŁdef".encode("cp1252", "surrogateescape"))
...
UnicodeEncodeError: 'charmap' codec can't encode character '\u0141' in position 3: character maps to <undefined>

[vstinner]

Close this issue: PEP-383 is specific to filesystem using bytes, it is useless on Windows (the problem on Windows is on encoding, not on decoding).