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Thu Jan 16 17:41:03 CET 2014 · https://mail.python.org/pipermail/python-dev/2014-January/131703.html

On Thu, Jan 16, 2014 at 11:33 AM, Michael Urman <murman at gmail.com> wrote:

> On Thu, Jan 16, 2014 at 8:45 AM, Brett Cannon <brett at python.org> wrote:
> > Fine, if you're worried about bytes.format() overstepping by implicitly
> > calling str.encode() on the return value of __format__() then you will
> need
> > __bytes__format__() to get equivalent support.
>
> Could we just re-use PEP-3101's note (easily updated for Python 3):
>
>     Note for Python 2.x: The 'format_spec' argument will be either
>     a string object or a unicode object, depending on the type of the
>     original format string.  The __format__ method should test the type
>     of the specifiers parameter to determine whether to return a string or
>     unicode object.  It is the responsibility of the __format__ method
>     to return an object of the proper type.
>
> If __format__ receives a format_spec of type bytes, it should return
> bytes. For such cases on objects that cannot support bytes (i.e. for
> str), it can raise. This appears to avoid the need for additional
> methods. (As does Nick's proposal of leaving it out for now.)
>

That's a very good catch, Michael! I think that makes sense if there is
precedence. Unfortunately that bit from the PEP never made it into the
documentation so I'm not sure if there is a backwards-compatibility worry.
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[Python-Dev] PEP 461 - Adding % and {} formatting to bytes