[Python-Dev] PEP 461: Adding % formatting to bytes and bytearray -- Final, Take 3 (original) (raw)

Ethan Furman ethan at stoneleaf.us
Thu Mar 27 19:04:12 CET 2014

On 03/27/2014 10:55 AM, Ethan Furman wrote:

On 03/27/2014 10:29 AM, Guido van Rossum wrote:

I also don't understand why we can't use %b instead of %s. AFAIK %b currently doesn't mean anything and I somehow don't expect we're likely to add it for other reasons (unless there's a proposal I'm missing?). Just like we use %a instead of %r to remind people that it's not quite the same (since it applies .encode('ascii', 'backslashreplace')), shouldn't we use anything but %s to remind people that that is also not the same (not at all, in fact)? The PEP's argument against %b ("rejected as not adding any value either in clarity or simplicity") is hardly a good reason. The biggest reason to use %s is to support a common code base for 2/3 endeavors. The biggest reason to not include %b is that it means binary number in format(); given that each type can invent it's own mini-language, this probably isn't a very strong argument against it. I have moderate feelings for keeping %s as a synonym for %b for backwards compatibility with Py2 code (when it's appropriate).

Changed to:

%b will insert a series of bytes. These bytes are collected in one of two ways:

In particular, %b will not accept numbers nor str. str is rejected as the string to bytes conversion requires an encoding, and we are refusing to guess; numbers are rejected because:

%s is included as a synonym for %b for the sole purpose of making 2/3 code bases easier to maintain. Python 3 only code should use %b.

Examples::

 >>> b'%b' % b'abc'
 b'abc'

 >>> b'%b' % 'some string'.encode('utf8')
 b'some string'

 >>> b'%b' % 3.14
 Traceback (most recent call last):
 ...
 TypeError: b'%b' does not accept 'float'

 >>> b'%b' % 'hello world!'
 Traceback (most recent call last):
 ...
 TypeError: b'%b' does not accept 'str'

-- Ethan

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