Duh. Here's the text, as well. ;)
PEP: 461
Title: Adding % and {} formatting to bytes
Version: $Revision$
Last-Modified: $Date$
Author: Ethan Furman <[email protected]>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 2014年01月13日
Python-Version: 3.5
Post-History: 2014年01月13日
Resolution:
Abstract
========
This PEP proposes adding the % and {} formatting operations from str to bytes.
Proposed semantics for bytes formatting
=======================================
%-interpolation
---------------
All the numeric formatting codes (such as %x, %o, %e, %f, %g, etc.)
will be supported, and will work as they do for str, including the
padding, justification and other related modifiers.
Example::
>>> b'%4x' % 10
b' a'
%c will insert a single byte, either from an int in range(256), or from
a bytes argument of length 1.
Example:
>>> b'%c' % 48
b'0'
>>> b'%c' % b'a'
b'a'
%s, because it is the most general, has the most convoluted resolution:
- input type is bytes?
pass it straight through
- input type is numeric?
use its __xxx__ [1] [2] method and ascii-encode it (strictly)
- input type is something else?
use its __bytes__ method; if there isn't one, raise an exception [3]
Examples:
>>> b'%s' % b'abc'
b'abc'
>>> b'%s' % 3.14
b'3.14'
>>> b'%s' % 'hello world!'
Traceback (most recent call last):
...
TypeError: 'hello world' has no __bytes__ method, perhaps you need to
encode it?
.. note::
Because the str type does not have a __bytes__ method, attempts to
directly use 'a string' as a bytes interpolation value will raise an
exception. To use 'string' values, they must be encoded or otherwise
transformed into a bytes sequence::
'a string'.encode('latin-1')
format
------
The format mini language will be used as-is, with the behaviors as listed
for %-interpolation.
Open Questions
==============
For %s there has been some discussion of trying to use the buffer protocol
(Py_buffer) before trying __bytes__. This question should be answered before
the PEP is implemented.
Proposed variations
===================
It has been suggested to use %b for bytes instead of %s.
- Rejected as %b does not exist in Python 2.x %-interpolation, which is
why we are using %s.
It has been proposed to automatically use .encode('ascii','strict') for str
arguments to %s.
- Rejected as this would lead to intermittent failures. Better to have the
operation always fail so the trouble-spot can be correctly fixed.
It has been proposed to have %s return the ascii-encoded repr when the value
is a str (b'%s' % 'abc' --> b"'abc'").
- Rejected as this would lead to hard to debug failures far from the problem
site. Better to have the operation always fail so the trouble-spot can be
easily fixed.
Foot notes
==========
.. [1] Not sure if this should be the numeric __str__ or the numeric __repr__,
or if there's any difference
.. [2] Any proper numeric class would then have to provide an ascii
representation of its value, either via __repr__ or __str__ (whichever
we choose in [1]).
.. [3] TypeError, ValueError, or UnicodeEncodeError?
Copyright
=========
This document has been placed in the public domain.
..
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