[issue3982] support .format for bytes

[Augie Fackler]

Augie Fackler added the comment:

On Oct 8, 2013, at 5:24 PM, STINNER Victor <report at bugs.python.org> wrote:

> 
> STINNER Victor added the comment:
> 
> 2013/10/8 Augie Fackler <report at bugs.python.org>:
>> sys.stdout.write('%(state)s %(path)s\n' % {'state': 'M', 'path':
>> 'some/filesystem/path'})
>> 
>> except we don't know the encoding of the filesystem path (Hi unix!) so we
>> have to treat the whole thing as opaque bytes.
> 
> You are doing it wrong. In Python 3, you "should" store filenames as
> Unicode (str type). If Python fails to decode a filename, undecodable
> bytes are stored as surrogate characters (see the PEP 383).

No, I'm not. In Mercurial, all end-user data is OPAQUE BYTES, and must remain that way. We're not able to change either our on-disk data format OR our stdout format, even to support a newer version of Python. I don't know the encoding of the filename's bytes, but I _must_ faithfully reproduce them exactly as they are or I'll break tools like make(1) and patch(1). Similarly, if a file goes from ISO-8859-1 to UTF-8, I have to emit a diff that has some ISO bytes and some UTF bytes - it's not in *any* valid encoding. Changing that is a showstopper regression.

> The Unicode type became natural in Python 3, as byte string (old "str"
> type) was natural in Python 2.
> 
> sys.stdout.write() expects a Unicode string, not a byte string.

Ouch. Is there any way to write things to stderr and stdout without decoding and hopelessly breaking user data?

> Does it mean that Mercurial is moving to Python 3? Cool :-)

Not likely, honestly. I tackle this when I've got some spare cycles and my ability to handle pain is high. As it stands, I have the test-runner barely working, but it's making wrong assumptions to get there. The best estimate is that it's a year of work to upgrade to Python 3.

> 
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