[Python-Dev] PEP 461: Adding % formatting to bytes and bytearray -- Final, Take 2

Nick Coghlan ncoghlan at gmail.com
Sun Feb 23 07:45:19 CET 2014

Thanks Ethan, this mostly looks excellent.

On 23 February 2014 11:56, Ethan Furman <ethan at stoneleaf.us> wrote:
> ``%a`` will call :func:``ascii()`` on the interpolated value's
> :func:``repr()``.
> This is intended as a debugging aid, rather than something that should be
> used
> in production.  Non-ascii values will be encoded to either ``\xnn`` or
> ``\unnnn``
> representation.

Is this really what is intended? It seems to me that what is needed is
to just call ascii(), which is inherently based on repr():

    >>> ascii(1)
    >>> ascii("1")
    >>> ascii(b"1")

The current wording in the PEP effectively suggests invoking repr()
twice, which is just weird:

    >>> ascii(repr(1))
    >>> ascii(repr("1"))
    >>> ascii(repr(b"1"))

And inconsistent with the meaning of %a in text interpolation:

>>> ("%a" % 1).encode("ascii")

> Open Questions
> ==============
> It has been suggested to use ``%b`` for bytes as well as ``%s``.
>   - Pro: clearly says 'this is bytes'; should be used for new code.
>   - Con: does not exist in Python 2.x, so we would have two ways of doing
> the
>     same thing, ``%s`` and ``%b``, with no difference between them.

Another con is that using %b this way would be inconsistent with the
"b" numeric format code that requests binary output:

>>> format(2, "b")
>>> format(2, "#b")

So -1 for "%b" from me on both TOOWTDI and consistency grounds.


Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia

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