Issue 19676: Add the "namereplace" error handler

The proposed patch adds the "namereplace" error handler. This error handler is almost same as the "backslashreplace" error handler, but use \N{...} escape sequences if there is a character name in Unicode database. Result is a little more human-readable (but less portable) than with "backslashreplace".

>>> '∀ x∈ℜ'.encode('ascii', 'namereplace')
b'\\N{FOR ALL} x\\N{ELEMENT OF}\\N{BLACK-LETTER CAPITAL R}'

The proposition was discussed and bikeshedded on Python-Ideas: http://comments.gmane.org/gmane.comp.python.ideas/21296 .

See also issue #18234.

Ping.

The patch looks good to me.
But it seems that the reverse operation is not possible in the general case: .decode('unicode_escape') assumes a latin-1 or ascii encoding.
Should we document this?

The patch looks good.

One nit: the name buffer length should be NAME_MAXLEN instead of 100.

Patch looks good to me, too.

As far as Amaury's question goes, isn't the general reverse operation the same as for the existing backslashreplace handler?

That is, decode with the appropriate ASCII compatible encoding (since ASCII compatibility is needed for the escape sequences to be valid), then run the result through ast.literal_eval?

(I'll grant we don't currently provide guidance on reversing backslashreplace either, but addressing that sounds like a separate question from this change)

New changeset 32d08aacffe0 by Serhiy Storchaka in branch 'default':
Issue #19676: Added the "namereplace" error handler.
https://hg.python.org/cpython/rev/32d08aacffe0

Thank you all for reviews.

> One nit: the name buffer length should be NAME_MAXLEN instead of 100.

NAME_MAXLEN is private name available only in Modules/unicodedata.c. Making it public name would be other issue. I have increased buffer size to 256.

New changeset b6fab008d63a by Berker Peksag in branch 'default':
Issue #19676: Tweak documentation a bit.
https://hg.python.org/cpython/rev/b6fab008d63a

New changeset 21d1571c0533 by Serhiy Storchaka in branch 'default':
Issue #19676: Fixed integer overflow issue in "namereplace" error handler.
https://hg.python.org/cpython/rev/21d1571c0533

Thank you Berker.

../../source/Python/codecs.c:1022:16: error: use of undeclared identifier 'out'; did you
      mean 'outp'?
        assert(out == start + ressize);
               ^~~
               outp

Fixed in ce8a8531d29a

stage: patch review -> resolved

versions: + Python 3.5, - Python 3.4