[Python-ideas] Support WHATWG versions of legacy encodings

M.-A. Lemburg mal at egenix.com
Fri Jan 19 08:30:31 EST 2018

On 19.01.2018 05:38, Nathaniel Smith wrote:
> On Thu, Jan 18, 2018 at 7:51 PM, Guido van Rossum <guido at python.org> wrote:
>> Can someone explain to me why this is such a controversial issue?
> I guess practicality versus purity is always controversial :-)
>> It seems reasonable to me to add new encodings to the stdlib that do the
>> roundtripping requested in the first message of the thread. As long as they
>> have new names that seems to fall under "practicality beats purity".

There are a few issues here:

* WHATWG encodings are mostly for decoding content in order to
  show it in the browser, accepting broken encoding data.

  Python already has support for this by using one of the available
  error handlers, or adding new ones to suit the needs.

  If we'd add the encodings, people will start creating more
  broken data, since this is what the WHATWG codecs output
  when encoding Unicode.

  As discussed, this could be addressed by making the WHATWG
  codecs decode-only.

* The use case seems limited to implementing browsers or headless
  implementations working like browsers.

  That's not really general enough to warrant adding lots of
  new codecs to the stdlib. A PyPI package is better suited
  for this.

* The WHATWG codecs do not only cover simple mapping codecs,
  but also many multi-byte ones for e.g. Asian languages.

  I doubt that we'd want to maintain such codecs in the stdlib,
  since this will increase the download sizes of the installers
  and also require people knowledgeable about these variants
  to work on them and fix any issues.

Overall, I think either pointing people to error handlers
or perhaps adding a new one specifically for the case of
dealing with control character mappings would provide a better
maintenance / usefulness ratio than adding lots of new
legacy codecs to the stdlib.

BTW: WHATWG pushes for always using UTF-8 as far as I can tell
from their website.

>> (Modifying existing encodings seems wrong -- did the feature request somehow
>> transmogrify into that?)
> Someone did discover that Microsoft's current implementations of the
> windows-* encodings matches the WHAT-WG spec, rather than the Unicode
> spec that Microsoft originally wrote.

No, MS implements somethings called "best fit encodings"
and these are different than what WHATWG uses.

Unlike the WHATWG encodings, these are documented as vendor encodings
on the Unicode site, which is what we normally use as reference
for out stdlib codecs.

However, whether these are actually a good idea, is open to discussion
as well, since they sometimes go a bit far with "best fit", e.g.
mapping the infinity symbol to 8.

Again, using the error handles we have for dealing with
situations which require non-standard encoding behavior are
the better approach:


Adding new ones is possible as well.

> So there is some argument that
> the Python's existing encodings are simply out of date, and changing
> them would be a bugfix. (And standards aside, it is surely going to be
> somewhat error-prone if Python's windows-1252 doesn't match everyone
> else's implementations of windows-1252.) But yeah, AFAICT the original
> requesters would be happy either way; they just want it available
> under some name.

The encodings are not out of date. I don't know where you got
that impression from.

The Windows API WideCharToMultiByte  which was quoted in the discussion:


unfortunately uses the above mentioned best fit encodings,
but this can and should be switched off by specifying the
WC_NO_BEST_FIT_CHARS for anything that requires validation
or needs to be interoperable:

For strings that require validation, such as file, resource, and user
names, the application should always use the WC_NO_BEST_FIT_CHARS flag.
This flag prevents the function from mapping characters to characters
that appear similar but have very different semantics. In some cases,
the semantic change can be extreme. For example, the symbol for "∞"
(infinity) maps to 8 (eight) in some code pages.

Marc-Andre Lemburg

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