218 lines
6.5 KiB
Plaintext
218 lines
6.5 KiB
Plaintext
PEP: 461
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Title: Adding % formatting to bytes and bytearray
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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: 2014-01-14, 2014-01-15, 2014-01-17, 2014-02-22
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Resolution:
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Abstract
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========
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This PEP proposes adding % formatting operations similar to Python 2's ``str``
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type to ``bytes`` and ``bytearray`` [1]_ [2]_.
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Rationale
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=========
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While interpolation is usually thought of as a string operation, there are
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cases where interpolation on ``bytes`` or ``bytearrays`` make sense, and the
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work needed to make up for this missing functionality detracts from the overall
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readability of the code.
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Motivation
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==========
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With Python 3 and the split between ``str`` and ``bytes``, one small but
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important area of programming became slightly more difficult, and much more
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painful -- wire format protocols [3]_.
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This area of programming is characterized by a mixture of binary data and
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ASCII compatible segments of text (aka ASCII-encoded text). Bringing back a
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restricted %-interpolation for ``bytes`` and ``bytearray`` will aid both in
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writing new wire format code, and in porting Python 2 wire format code.
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Overriding Principles
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=====================
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In order to avoid the problems of auto-conversion and Unicode exceptions
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that could plague Python 2 code, ``str`` objects will not be supported as
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interpolation values [4]_ [5]_.
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Proposed semantics for ``bytes`` and ``bytearray`` 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``,
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``%g``, etc.) will be supported, and will work as they do for str, including
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the 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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>>> '%#4x' % 10
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' 0xa'
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>>> '%04X' % 10
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'000A'
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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, not from a ``str``.
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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`` is restricted in what it will accept::
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- input type supports ``Py_buffer`` [6]_?
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use it to collect the necessary bytes
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- input type is something else?
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use its ``__bytes__`` method [7]_ ; if there isn't one, raise a ``TypeError``
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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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Traceback (most recent call last):
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...
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TypeError: 3.14 has no __bytes__ method, use a numeric code instead
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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 strings they must be encoded or otherwise transformed
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into a ``bytes`` sequence::
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'a string'.encode('latin-1')
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``%a`` will call ``ascii()`` on the interpolated value's ``repr()``.
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This is intended as a debugging aid, rather than something that should be used
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in production. Non-ascii values will be encoded to either ``\xnn`` or ``\unnnn``
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representation.
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Unsupported codes
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-----------------
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``%r`` (which calls ``__repr__`` and returns a '`str`') is not supported.
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Proposed variations
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===================
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It was suggested to let ``%s`` accept numbers, but since numbers have their own
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format codes this idea was discarded.
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It has been proposed to automatically use ``.encode('ascii','strict')`` for
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``str`` 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
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value 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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Originally this PEP also proposed adding format-style formatting, but it was
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decided that format and its related machinery were all strictly text (aka
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``str``) based, and it was dropped.
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Various new special methods were proposed, such as ``__ascii__``,
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``__format_bytes__``, etc.; such methods are not needed at this time, but can
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be visited again later if real-world use shows deficiencies with this solution.
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Objections
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==========
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The objections raised against this PEP were mainly variations on two themes::
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- the ``bytes`` and ``bytearray`` types are for pure binary data, with no
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assumptions about encodings
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- offering %-interpolation that assumes an ASCII encoding will be an
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attractive nuisance and lead us back to the problems of the Python 2
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``str``/``unicode`` text model
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As was seen during the discussion, ``bytes`` and ``bytearray`` are also used
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for mixed binary data and ASCII-compatible segments: file formats such as
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``dbf`` and ``pdf``, network protocols such as ``ftp`` and ``email``, etc.
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``bytes`` and ``bytearray`` already have several methods which assume an ASCII
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compatible encoding. ``upper()``, ``isalpha()``, and ``expandtabs()`` to name
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just a few. %-interpolation, with its very restricted mini-language, will not
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be any more of a nuisance than the already existing methdods.
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Open Questions
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==============
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It has been suggested to use ``%b`` for bytes as well as ``%s``.
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- Pro: clearly says 'this is bytes'; should be used for new code.
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- Con: does not exist in Python 2.x, so we would have two ways of doing the
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same thing, ``%s`` and ``%b``, with no difference between them.
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Footnotes
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=========
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.. [1] http://docs.python.org/2/library/stdtypes.html#string-formatting
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.. [2] neither string.Template, format, nor str.format are under consideration
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.. [3] https://mail.python.org/pipermail/python-dev/2014-January/131518.html
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.. [4] to use a str object in a bytes interpolation, encode it first
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.. [5] %c is not an exception as neither of its possible arguments are str
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.. [6] http://docs.python.org/3/c-api/buffer.html
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examples: ``memoryview``, ``array.array``, ``bytearray``, ``bytes``
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.. [7] http://docs.python.org/3/reference/datamodel.html#object.__bytes__
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.. [8] mainly implicit encode/decode, with intermittent errors when the data
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was not ASCII compatible
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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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