217 lines
7.4 KiB
Plaintext
217 lines
7.4 KiB
Plaintext
PEP: 467
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Title: Minor API improvements for binary sequences
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Version: $Revision$
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Last-Modified: $Date$
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Author: Nick Coghlan <ncoghlan@gmail.com>, 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: 2014-03-30
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Python-Version: 3.8
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Post-History: 2014-03-30 2014-08-15 2014-08-16 2016-06-07 2016-09-01
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Abstract
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========
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During the initial development of the Python 3 language specification, the
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core ``bytes`` type for arbitrary binary data started as the mutable type
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that is now referred to as ``bytearray``. Other aspects of operating in
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the binary domain in Python have also evolved over the course of the Python
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3 series.
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This PEP proposes five small adjustments to the APIs of the ``bytes`` and
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``bytearray`` types to make it easier to operate entirely in the binary domain:
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* Deprecate passing single integer values to ``bytes`` and ``bytearray``
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* Add ``bytes.fromsize`` and ``bytearray.fromsize`` alternative constructors
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* Add ``bytes.fromord`` and ``bytearray.fromord`` alternative constructors
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* Add ``bytes.getbyte`` and ``bytearray.getbyte`` byte retrieval methods
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* Add ``bytes.iterbytes`` and ``bytearray.iterbytes`` alternative iterators
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And one built-in::
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* bchr
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Proposals
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=========
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Deprecation of current "zero-initialised sequence" behaviour without removal
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----------------------------------------------------------------------------
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Currently, the ``bytes`` and ``bytearray`` constructors accept an integer
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argument and interpret it as meaning to create a zero-initialised sequence
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of the given size::
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>>> bytes(3)
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b'\x00\x00\x00'
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>>> bytearray(3)
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bytearray(b'\x00\x00\x00')
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This PEP proposes to deprecate that behaviour in Python 3.6, but to leave
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it in place for at least as long as Python 2.7 is supported, possibly
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indefinitely.
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No other changes are proposed to the existing constructors.
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Addition of explicit "count and byte initialised sequence" constructors
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-----------------------------------------------------------------------
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To replace the deprecated behaviour, this PEP proposes the addition of an
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explicit ``fromsize`` alternative constructor as a class method on both
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``bytes`` and ``bytearray`` whose first argument is the count, and whose
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second argument is the fill byte to use (defaults to ``\x00``)::
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>>> bytes.fromsize(3)
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b'\x00\x00\x00'
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>>> bytearray.fromsize(3)
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bytearray(b'\x00\x00\x00')
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>>> bytes.fromsize(5, b'\x0a')
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b'\x0a\x0a\x0a\x0a\x0a'
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>>> bytearray.fromsize(5, b'\x0a')
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bytearray(b'\x0a\x0a\x0a\x0a\x0a')
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``fromsize`` will behave just as the current constructors behave when passed a single
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integer, while allowing for non-zero fill values when needed.
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Addition of "bchr" function and explicit "single byte" constructors
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-------------------------------------------------------------------
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As binary counterparts to the text ``chr`` function, this PEP proposes
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the addition of a ``bchr`` function and an explicit ``fromord`` alternative
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constructor as a class method on both ``bytes`` and ``bytearray``::
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>>> bchr(ord("A"))
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b'A'
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>>> bchr(ord(b"A"))
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b'A'
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>>> bytes.fromord(65)
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b'A'
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>>> bytearray.fromord(65)
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bytearray(b'A')
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These methods will only accept integers in the range 0 to 255 (inclusive)::
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>>> bytes.fromord(512)
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Traceback (most recent call last):
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File "<stdin>", line 1, in <module>
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ValueError: integer must be in range(0, 256)
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>>> bytes.fromord(1.0)
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Traceback (most recent call last):
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File "<stdin>", line 1, in <module>
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TypeError: 'float' object cannot be interpreted as an integer
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While this does create some duplication, there are valid reasons for it:
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* the ``bchr`` builtin is to recreate the ord/chr/unichr trio from Python
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2 under a different naming scheme
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* the class method is mainly for the ``bytearray.fromord`` case, with
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``bytes.fromord`` added for consistency
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The documentation of the ``ord`` builtin will be updated to explicitly note
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that ``bchr`` is the primary inverse operation for binary data, while ``chr``
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is the inverse operation for text data, and that ``bytes.fromord`` and
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``bytearray.fromord`` also exist.
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Behaviourally, ``bytes.fromord(x)`` will be equivalent to the current
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``bytes([x])`` (and similarly for ``bytearray``). The new spelling is
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expected to be easier to discover and easier to read (especially when used
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in conjunction with indexing operations on binary sequence types).
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As a separate method, the new spelling will also work better with higher
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order functions like ``map``.
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Addition of "getbyte" method to retrieve a single byte
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------------------------------------------------------
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This PEP proposes that ``bytes`` and ``bytearray`` gain the method ``getbyte``
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which will always return ``bytes``::
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>>> b'abc'.getbyte(0)
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b'a'
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If an index is asked for that doesn't exist, ``IndexError`` is raised::
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>>> b'abc'.getbyte(9)
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Traceback (most recent call last):
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File "<stdin>", line 1, in <module>
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IndexError: index out of range
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Addition of optimised iterator methods that produce ``bytes`` objects
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---------------------------------------------------------------------
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This PEP proposes that ``bytes`` and ``bytearray``gain an optimised
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``iterbytes`` method that produces length 1 ``bytes`` objects rather than
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integers::
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for x in data.iterbytes():
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# x is a length 1 ``bytes`` object, rather than an integer
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For example::
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>>> tuple(b"ABC".iterbytes())
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(b'A', b'B', b'C')
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Design discussion
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=================
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Why not rely on sequence repetition to create zero-initialised sequences?
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-------------------------------------------------------------------------
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Zero-initialised sequences can be created via sequence repetition::
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>>> b'\x00' * 3
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b'\x00\x00\x00'
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>>> bytearray(b'\x00') * 3
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bytearray(b'\x00\x00\x00')
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However, this was also the case when the ``bytearray`` type was originally
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designed, and the decision was made to add explicit support for it in the
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type constructor. The immutable ``bytes`` type then inherited that feature
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when it was introduced in PEP 3137.
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This PEP isn't revisiting that original design decision, just changing the
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spelling as users sometimes find the current behaviour of the binary sequence
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constructors surprising. In particular, there's a reasonable case to be made
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that ``bytes(x)`` (where ``x`` is an integer) should behave like the
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``bytes.fromord(x)`` proposal in this PEP. Providing both behaviours as separate
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class methods avoids that ambiguity.
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Open Questions
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==============
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Do we add ``iterbytes`` to ``memoryview``, or modify
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``memoryview.cast()`` to accept ``'s'`` as a single-byte interpretation? Or
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do we ignore memory for now and add it later?
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References
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==========
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.. [1] Initial March 2014 discussion thread on python-ideas
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(https://mail.python.org/pipermail/python-ideas/2014-March/027295.html)
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.. [2] Guido's initial feedback in that thread
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(https://mail.python.org/pipermail/python-ideas/2014-March/027376.html)
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.. [3] Issue proposing moving zero-initialised sequences to a dedicated API
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(http://bugs.python.org/issue20895)
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.. [4] Issue proposing to use calloc() for zero-initialised binary sequences
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(http://bugs.python.org/issue21644)
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.. [5] August 2014 discussion thread on python-dev
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(https://mail.python.org/pipermail/python-ideas/2014-March/027295.html)
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.. [6] June 2016 discussion thread on python-dev
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(https://mail.python.org/pipermail/python-dev/2016-June/144875.html)
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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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