[Python-Dev] PEP 461 - Adding % and {} formatting to bytes

[Neil Schemenauer]

Glenn Linderman <v+python at g.nevcal.com> wrote:
> On 1/15/2014 7:52 AM, Eric V. Smith wrote:
>> Either that, or we're back to encoding the result of __format__ and
>> accepting that sometimes it might throw errors, depending on the values
>> being passed into format().

That would take us back to Python 2 hell.  Please no.  I don't like
checking for types either, we should have a special method.

> Looks like you need to invent  __formatb__ to produce only ASCII. 
> Objects that have __formatb__ can be formatted by bytes.format.  To 
> avoid coding, it could be possible that __formatb__ might be a callable
> in which case it is called to get the result, or not a callable, in 
> which case one calls __format__ and converts the result to ASCII, 
> __formatb__ just indicating a guarantee that only ASCII will result.

Just do:

    def __formatb__(self, spec):
        return MyClass.__format__(self, spec).encode('ascii')

Note that I think it is better to explicitly use the __format__
method rather than using self.__format__.  My reasoning is that a
subclass might implement a __format__ that returns non-ASCII
characters.

We don't need a special bytes version of __str__ since the
%-operator can call __formatb__ with the correct format spec.

   Neil