[Python-Dev] PEP 461 updates (original) (raw)
Ethan Furman ethan at stoneleaf.us
Thu Jan 16 01:13:55 CET 2014
- Previous message: [Python-Dev] PEP 461 - Adding % and {} formatting to bytes
- Next message: [Python-Dev] PEP 461 updates
- Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]
Current copy of PEP, many modifications from all the feedback. Thank you to everyone.
I know it's been a long week (feels a lot longer!) while all this was hammered out, but I think we're getting close!
============================ Abstract
This PEP proposes adding the % and {} formatting operations from str to bytes [1].
Overriding Principles
In order to avoid the problems of auto-conversion and value-generated exceptions, all object checking will be done via isinstance, not by values contained in a Unicode representation. In other words::
- duck-typing to allow/reject entry into a byte-stream
- no value generated errors
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, except locale.
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 is restricted in what it will accept::
input type supports Py_buffer? use it to collect the necessary bytes
input type is something else? use its bytes method; if there isn't one, raise an exception [2]
Examples:
>>> b'%s' % b'abc'
b'abc'
>>> b'%s' % 3.14
Traceback (most recent call last):
...
TypeError: 3.14 has no __bytes__ method
>>> 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 codes, where they correspond with the %-interpolation codes, will be used as-is, with three exceptions::
- !s is not supported, as {} can mean the default for both str and bytes, in both Py2 and Py3.
- !b is supported, and new Py3k code can use it to be explicit.
- no other format method will be called.
Numeric Format Codes
To properly handle int and float subclasses, int(), index(), and float() will be called on the objects intended for (d, i, u), (b, o, x, X), and (e, E, f, F, g, G).
Unsupported codes
%r (which calls repr), and %a (which calls ascii() on repr) are not supported.
!r and !a are not supported.
The n integer and float format code is not supported.
Open Questions
Currently non-numeric objects go through::
- Py_buffer
- bytes
- failure
Do we want to add a format_bytes method in there?
- Guaranteed to produce only ascii (as in b'10', not b'\x0a')
- Makes more sense than using bytes to produce ascii output
- What if an object has both bytes and format_bytes?
Do we need to support all the numeric format codes? The floating point exponential formats seem less appropriate, for example.
Proposed variations
It was suggested to let %s accept numbers, but since numbers have their own format codes this idea was discarded.
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.
Footnotes
.. [1] string.Template is not under consideration. .. [2] TypeError, ValueError, or UnicodeEncodeError?
--
Ethan
- Previous message: [Python-Dev] PEP 461 - Adding % and {} formatting to bytes
- Next message: [Python-Dev] PEP 461 updates
- Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]