[Python-ideas] a new bytestring type?

[Geert Jansen]

On Mon, Jan 6, 2014 at 12:34 PM, Andrew Barnert <abarnert at yahoo.com> wrote:

> b'{}'.format(x) can't call bytes(x). At least not unless you want b'#{}'.format(6) to give you b'#\0\0\0\0\0\0'. Besides, most types don't provide a __bytes__, so even if it weren't for this problem, it wouldn't really be useful for anything except inserting bytes into other bytes. So, what _should_ it call?
>
> You could add encoding and errors keyword parameters (defaulting to 'ascii' and 'strict'), so b'{}'.format(x, encoding='utf-8') calls str(x).encode('utf-8'), which solves all of those problems… except that now it means you can't stick bytes objects into bytes formats, which is even worse.
>
> You could solve that by making objects that support the buffer protocol (like bytes) copy as-is instead of going through str and encode. That would mean you can't use bytes with a placeholder with any format flags, but maybe that's a good thing anyway (e.g., do you really want b'{:3}'.format(b'\xc3\xa9') to only pad to 2 characters instead of 3 because it's a 2-byte character?).
>
> That would be enough to let you cram pre-encoded/formatted bytes, and things like numbers, into bytes formats made up of ASCII headers, which I think is 90% of what people want here. Does that seem worth pursuing?

Agreed that probably the main case is inserting bytes objects verbatim
in a message with a a small ASCII header and possibly trainer. Format
flags are useful, e.g. with chunked HTTP encoding you need to insert
the length in hex. But if those are only available for non-bytes
objects that'd probably be fine.

I'm not too familiar with the implementation of format() so I can't
say much about it.

Regards,
Geert