<div dir="ltr">bytes.format() below. I'll leave it to you to decide if they warrant using, leaving as an open question, or rejecting.<br><div class="gmail_extra"><br><br><div class="gmail_quote">On Tue, Jan 14, 2014 at 2:56 PM, Ethan Furman <span dir="ltr"><<a href="mailto:ethan@stoneleaf.us" target="_blank">ethan@stoneleaf.us</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Duh. Here's the text, as well. ;)<br>
<br>
<br>
PEP: 461<br>
Title: Adding % and {} formatting to bytes<br>
Version: $Revision$<br>
Last-Modified: $Date$<br>
Author: Ethan Furman <<a href="mailto:ethan@stoneleaf.us" target="_blank">ethan@stoneleaf.us</a>><br>
Status: Draft<br>
Type: Standards Track<br>
Content-Type: text/x-rst<br>
Created: 2014-01-13<br>
Python-Version: 3.5<br>
Post-History: 2014-01-13<br>
Resolution:<br>
<br>
<br>
Abstract<br>
========<br>
<br>
This PEP proposes adding the % and {} formatting operations from str to bytes.<br>
<br>
<br>
Proposed semantics for bytes formatting<br>
==============================<u></u>=========<br>
<br>
%-interpolation<br>
---------------<br>
<br>
All the numeric formatting codes (such as %x, %o, %e, %f, %g, etc.)<br>
will be supported, and will work as they do for str, including the<br>
padding, justification and other related modifiers.<br>
<br>
Example::<br>
<br>
>>> b'%4x' % 10<br>
b' a'<br>
<br>
%c will insert a single byte, either from an int in range(256), or from<br>
a bytes argument of length 1.<br>
<br>
Example:<br>
<br>
>>> b'%c' % 48<br>
b'0'<br>
<br>
>>> b'%c' % b'a'<br>
b'a'<br>
<br>
%s, because it is the most general, has the most convoluted resolution:<br>
<br>
- input type is bytes?<br>
pass it straight through<br>
<br>
- input type is numeric?<br>
use its __xxx__ [1] [2] method and ascii-encode it (strictly)<br>
<br>
- input type is something else?<br>
use its __bytes__ method; if there isn't one, raise an exception [3]<br>
<br>
Examples:<br>
<br>
>>> b'%s' % b'abc'<br>
b'abc'<br>
<br>
>>> b'%s' % 3.14<br>
b'3.14'<br>
<br>
>>> b'%s' % 'hello world!'<br>
Traceback (most recent call last):<br>
...<br>
TypeError: 'hello world' has no __bytes__ method, perhaps you need to encode it?<br>
<br>
.. note::<br>
<br>
Because the str type does not have a __bytes__ method, attempts to<br>
directly use 'a string' as a bytes interpolation value will raise an<br>
exception. To use 'string' values, they must be encoded or otherwise<br>
transformed into a bytes sequence::<br>
<br>
'a string'.encode('latin-1')<br>
<br>
<br>
format<br>
------<br>
<br>
The format mini language will be used as-is, with the behaviors as listed<br>
for %-interpolation.<br></blockquote><div><br></div><div>That's too vague; % interpolation does not support other format operators in the same way as str.format() does. % interpolation has specific code to support %d, etc. But str.format() gets supported for {:d} not from special code but because e.g. float.__format__('d') works. So you can't say "bytes.format() supports {:d} just like %d works with string interpolation" since the mechanisms are fundamentally different.</div>
<div><br></div><div>This is why I have argued that if you specify it as "if there is a format spec specified, then the return value from calling __format__() will have str.decode('ascii', 'strict') called on it" you get the support for the various number-specific format specs for free. It also means if you pass in a string that you just want the strict ASCII bytes of then you can get it with {:s}.</div>
<div><br></div><div>I also think that a 'b' conversion be added to bytes.format(). This doesn't have the same issue as %b if you make {} implicitly mean {!b} in Python 3.5 as {} will mean what is the most accurate for bytes.format() in either version. It also allows for explicit support where you know you only want a byte and allows {!s} to mean you only want a string (and thus throw an error otherwise).</div>
<div><br></div><div>And all of this means that much like %s only taking bytes, the only way for bytes.format() to accept a non-byte argument is for some format spec to be specified to trigger the .encode('ascii', 'strict') call.</div>
<div><br></div><div>-Brett</div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<br>
<br>
Open Questions<br>
==============<br>
<br>
For %s there has been some discussion of trying to use the buffer protocol<br>
(Py_buffer) before trying __bytes__. This question should be answered before<br>
the PEP is implemented.<br>
<br>
<br>
Proposed variations<br>
===================<br>
<br>
It has been suggested to use %b for bytes instead of %s.<br>
<br>
- Rejected as %b does not exist in Python 2.x %-interpolation, which is<br>
why we are using %s.<br>
<br>
It has been proposed to automatically use .encode('ascii','strict') for str<br>
arguments to %s.<br>
<br>
- Rejected as this would lead to intermittent failures. Better to have the<br>
operation always fail so the trouble-spot can be correctly fixed.<br>
<br>
It has been proposed to have %s return the ascii-encoded repr when the value<br>
is a str (b'%s' % 'abc' --> b"'abc'").<br>
<br>
- Rejected as this would lead to hard to debug failures far from the problem<br>
site. Better to have the operation always fail so the trouble-spot can be<br>
easily fixed.<br>
<br>
<br>
Foot notes<br>
==========<br>
<br>
.. [1] Not sure if this should be the numeric __str__ or the numeric __repr__,<br>
or if there's any difference<br>
.. [2] Any proper numeric class would then have to provide an ascii<br>
representation of its value, either via __repr__ or __str__ (whichever<br>
we choose in [1]).<br>
.. [3] TypeError, ValueError, or UnicodeEncodeError?<br>
<br>
<br>
Copyright<br>
=========<br>
<br>
This document has been placed in the public domain.<br>
<br>
<br>
..<br>
Local Variables:<br>
mode: indented-text<br>
indent-tabs-mode: nil<br>
sentence-end-double-space: t<br>
fill-column: 70<br>
coding: utf-8<br>
End:<div class="HOEnZb"><div class="h5"><br>
<br>
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</div></div></blockquote></div><br></div></div>