[Python-ideas] Support WHATWG versions of legacy encodings
M.-A. Lemburg
mal at egenix.com
Wed Jan 10 03:38:42 EST 2018
On 10.01.2018 00:56, Rob Speer wrote:
> Oh that's interesting. So it seems to be Python that's the exception here.
>
> Would we really be able to add entries to character mappings that haven't
> changed since Python 2.0?
The Windows mappings in Python come directly from the Unicode
Consortium mapping files.
If the Consortium changes the mappings, we can update them.
If not, then we have a problem, since consumers are not only
the win32 APIs, but also other tools out there running on
completely different platforms, e.g. Java tools or web servers
providing downloads using the Windows code page encodings.
Allowing such mappings in the existing codecs would then result
failures when the "other" sides see the decoded Unicode version and
try to encode back into the original encoding - you'd move the
problem from the Python side to the "other" side of the
integration.
I had a look on the Unicode FTP site and they have since added
a new directory with mapping files they call "best fit":
http://www.unicode.org/Public/MAPPINGS/VENDORS/MICSFT/WindowsBestFit/readme.txt
The WideCharToMultiByte() defaults to best fit, but also offers
a mode where it operates in standards compliant mode:
https://msdn.microsoft.com/en-us/library/windows/desktop/dd374130%28v=vs.85%29.aspx
See flag WC_NO_BEST_FIT_CHARS.
Unicode TR#22 is also clear on this:
https://www.unicode.org/reports/tr22/tr22-3.html#Illegal_and_Unassigned
It allows such best fit mappings to make encodings round-trip
safe, but requires to keep these separate from the original
standard mappings:
"""
It is very important that systems be able to distinguish between the
fallback mappings and regular mappings. Systems like XML require the use
of hex escape sequences (NCRs) to preserve round-trip integrity; use of
fallback characters in that case corrupts the data.
"""
If you read the above section in TR#22 you quickly get reminded
of what the Unicode error handlers do (we basically implement
the three modes it mentions... raise, ignore, replace).
Now, for unmapped sequences an error handler can opt for
using a fallback sequence instead.
So in addition to adding best fit codecs, there's also the
option to add an error handler for best fit resolution of
unmapped sequences.
Given the above, I don't think we ought to change the existing
standards compliant mappings, but use one of two solutions:
a) add "best fit" encodings (see the Unicode FTP site for
a list)
b) add an error handlers "bestfit" which implements the
fallback modes for the encodings in question
> On Tue, 9 Jan 2018 at 16:53 Ivan Pozdeev via Python-ideas <
> python-ideas at python.org> wrote:
>
>> First of all, many thanks for such a excellently writen letter. It was a
>> real pleasure to read.
>> On 10.01.2018 0:15, Rob Speer wrote:
>>
>> Hi! I joined this list because I'm interested in filling a gap in Python's
>> standard library, relating to text encodings.
>>
>> There is an encoding with no name of its own. It's supported by every
>> current web browser and standardized by WHATWG. It's so prevalent that if
>> you ask a Web browser to decode "iso-8859-1" or "windows-1252", you will
>> get this encoding _instead_. It is probably the second or third most common
>> text encoding in the world. And Python doesn't quite support it.
>>
>> You can see the character table for this encoding at:
>> https://encoding.spec.whatwg.org/index-windows-1252.txt
>>
>> For the sake of discussion, let's call this encoding "web-1252". WHATWG
>> calls it "windows-1252", but notice that it's subtly different from
>> Python's "windows-1252" encoding. Python's windows-1252 has bytes that are
>> undefined:
>>
>>>>> b'\x90'.decode('windows-1252')
>> UnicodeDecodeError: 'charmap' codec can't decode byte 0x90 in position 0:
>> character maps to <undefined>
>>
>> In web-1252, the bytes that are undefined according to windows-1252 map to
>> the control characters in those positions in iso-8859-1 -- that is, the
>> Unicode codepoints with the same number as the byte. In web-1252, b'\x90'
>> would decode as '\u0090'.
>>
>> According to https://en.wikipedia.org/wiki/Windows-1252 , Windows does
>> the same:
>>
>> "According to the information on Microsoft's and the Unicode
>> Consortium's websites, positions 81, 8D, 8F, 90, and 9D are unused;
>> however, the Windows API MultiByteToWideChar
>> <http://msdn.microsoft.com/en-us/library/windows/desktop/dd319072%28v=vs.85%29.aspx>
>> maps these to the corresponding C1 control codes
>> <https://en.wikipedia.org/wiki/C0_and_C1_control_codes>."
>> And in ISO-8859-1, the same handling is done for unused code points even
>> by the standard ( https://en.wikipedia.org/wiki/ISO/IEC_8859-1 ) :
>>
>> "*ISO-8859-1* is the IANA
>> <https://en.wikipedia.org/wiki/Internet_Assigned_Numbers_Authority>
>> preferred name for this standard when supplemented with the C0 and C1
>> control codes <https://en.wikipedia.org/wiki/C0_and_C1_control_codes>
>> from ISO/IEC 6429 <https://en.wikipedia.org/wiki/ISO/IEC_6429>"
>> And what would you think -- these "C1 control codes" are also the
>> corresponding Unicode points! (
>> https://en.wikipedia.org/wiki/Latin-1_Supplement_(Unicode_block) )
>>
>> Since Windows is pretty much the reference implementation for
>> "windows-xxxx" encodings, it even makes sense to alter the existing
>> encodings rather than add new ones.
>>
>>
>> This may seem like a silly encoding that encourages doing horrible things
>> with text. That's pretty much the case. But there's a reason every Web
>> browser implements it:
>>
>> - It's compatible with windows-1252
>> - Any sequence of bytes can be round-tripped through it without losing
>> information
>>
>> It's not just this one encoding. WHATWG's encoding standard (
>> https://encoding.spec.whatwg.org/) contains modified versions of
>> windows-1250 through windows-1258 and windows-874.
>>
>> Support for these encodings matters to me, in part, because I maintain a
>> Unicode data-cleaning library, "ftfy". One thing it does is to detect and
>> undo encoding/decoding errors that cause mojibake, as long as they're
>> detectible and reversible. Looking at real-world examples of text that has
>> been damaged by mojibake, it's clear that lots of text is transferred
>> through what I'm calling the "web-1252" encoding, in a way that's
>> incompatible with Python's "windows-1252".
>>
>> In order to be able to work with and fix this kind of text, ftfy registers
>> new codecs -- and I implemented this even before I knew that they were
>> standardized in Web browsers. When ftfy is imported, you can decode text as
>> "sloppy-windows-1252" (the name I chose for this encoding), for example.
>>
>> ftfy can tell people a sequence of steps that they can use in the future
>> to fix text that's like the text they provided. Very often, these steps
>> require the sloppy-windows-1252 or sloppy-windows-1251 encoding, which
>> means the steps only work with ftfy imported, even for people who are not
>> using the features of ftfy.
>>
>> Support for these encodings also seems highly relevant to people who use
>> Python for web scraping, as it would be desirable to maximize compatibility
>> with what a Web browser would do.
>>
>> This really seems like it belongs in the standard library instead of being
>> an incidental feature of my library. I know that code in the standard
>> library has "one foot in the grave". I _want_ these legacy encodings to
>> have one foot in the grave. But some of them are extremely common, and
>> Python code should be able to deal with them.
>>
>> Adding these encodings to Python would be straightforward to implement.
>> Does this require a PEP, a pull request, or further discussion?
>>
>>
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>>
>> --
>> Regards,
>> Ivan
>>
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