143 lines
3.3 KiB
Plaintext
143 lines
3.3 KiB
Plaintext
PEP: 460
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Title: Adding % and {} formatting to bytes
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Version: $Revision$
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Last-Modified: $Date$
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Author: Ethan Furman <ethan@stoneleaf.us>
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Status: Draft
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Type: Standards Track
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Content-Type: text/x-rst
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Created: 2014-01-13
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Python-Version: 3.5
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Post-History: 2014-01-13
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Resolution:
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Abstract
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========
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This PEP proposes adding the % and {} formatting operations from str to bytes.
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Proposed semantics for bytes formatting
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=======================================
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%-interpolation
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---------------
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All the numeric formatting codes (such as %x, %o, %e, %f, %g, etc.)
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will be supported, and will work as they do for str, including the
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padding, justification and other related modifiers.
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Example::
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>>> b'%4x' % 10
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b' a'
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%c will insert a single byte, either from an int in range(256), or from
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a bytes argument of length 1.
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Example:
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>>> b'%c' % 48
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b'0'
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>>> b'%c' % b'a'
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b'a'
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%s, because it is the most general, has the most convoluted resolution:
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- input type is bytes?
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pass it straight through
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- input type is numeric?
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use its __xxx__ [1] [2] method and ascii-encode it (strictly)
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- input type is something else?
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use its __bytes__ method; if there isn't one, raise an exception [3]
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Examples:
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>>> b'%s' % b'abc'
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b'abc'
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>>> b'%s' % 3.14
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b'3.14'
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>>> b'%s' % 'hello world!'
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Traceback (most recent call last):
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...
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TypeError: 'hello world' has no __bytes__ method, perhaps you need to encode it?
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.. note::
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Because the str type does not have a __bytes__ method, attempts to
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directly use 'a string' as a bytes interpolation value will raise an
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exception. To use 'string' values, they must be encoded or otherwise
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transformed into a bytes sequence::
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'a string'.encode('latin-1')
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format
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------
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The format mini language will be used as-is, with the behaviors as listed
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for %-interpolation.
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Open Questions
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==============
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For %s there has been some discussion of trying to use the buffer protocol
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(Py_buffer) before trying __bytes__. This question should be answered before
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the PEP is implemented.
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Proposed variations
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===================
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It has been suggested to use %b for bytes instead of %s.
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- Rejected as %b does not exist in Python 2.x %-interpolation, which is
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why we are using %s.
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It has been proposed to automatically use .encode('ascii','strict') for str
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arguments to %s.
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- Rejected as this would lead to intermittent failures. Better to have the
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operation always fail so the trouble-spot can be correctly fixed.
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It has been proposed to have %s return the ascii-encoded repr when the value
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is a str (b'%s' % 'abc' --> b"'abc'").
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- Rejected as this would lead to hard to debug failures far from the problem
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site. Better to have the operation always fail so the trouble-spot can be
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easily fixed.
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Foot notes
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==========
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.. [1] Not sure if this should be the numeric __str__ or the numeric __repr__,
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or if there's any difference
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.. [2] Any proper numeric class would then have to provide an ascii
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representation of its value, either via __repr__ or __str__ (whichever
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we choose in [1]).
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.. [3] TypeError, ValueError, or UnicodeEncodeError?
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Copyright
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=========
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This document has been placed in the public domain.
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..
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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sentence-end-double-space: t
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fill-column: 70
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coding: utf-8
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End:
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