python-peps/pep-0467.txt

PEP: 467
Title: Minor API improvements for binary sequences
Version: $Revision$
Last-Modified: $Date$
Author: Nick Coghlan <ncoghlan@gmail.com>, Ethan Furman <ethan@stoneleaf.us>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 30-Mar-2014
Python-Version: 3.11
Post-History: 2014-03-30 2014-08-15 2014-08-16 2016-06-07 2016-09-01 2021-04-13


Abstract
========

This PEP proposes five small adjustments to the APIs of the ``bytes`` and
``bytearray`` types to make it easier to operate entirely in the binary domain:

* Add ``fromsize`` alternative constructor
* Add ``fromint`` alternative constructor
* Add ``ascii`` alternative constructor
* Add ``getbyte`` byte retrieval method
* Add ``iterbytes`` alternative iterator

Rationale
=========

During the initial development of the Python 3 language specification, the
core ``bytes`` type for arbitrary binary data started as the mutable type
that is now referred to as ``bytearray``. Other aspects of operating in
the binary domain in Python have also evolved over the course of the Python
3 series, for example with PEP 461.


Motivation
==========

With Python 3 and the split between ``str`` and ``bytes``, one small but
important area of programming became slightly more difficult, and much more
painful -- wire format protocols.

This area of programming is characterized by a mixture of binary data and
ASCII compatible segments of text (aka ASCII-encoded text).  The addition of
the new constructors, methods, and iterators will aid both in writing new
wire format code, and in porting any remaining Python 2 wire format code.

Common use-cases include ``dbf`` and ``pdf`` file formats, ``email``
formats, and ``FTP`` and ``HTTP`` communications, among many others.


Proposals
=========

Addition of explicit "count and byte initialised sequence" constructors
-----------------------------------------------------------------------

To replace the now discouraged behavior, this PEP proposes the addition of an
explicit ``fromsize`` alternative constructor as a class method on both
``bytes`` and ``bytearray`` whose first argument is the count, and whose
second argument is the fill byte to use (defaults to ``\x00``)::

    >>> bytes.fromsize(3)
    b'\x00\x00\x00'
    >>> bytearray.fromsize(3)
    bytearray(b'\x00\x00\x00')
    >>> bytes.fromsize(5, b'\x0a')
    b'\x0a\x0a\x0a\x0a\x0a'
    >>> bytearray.fromsize(5, fill=b'\x0a')
    bytearray(b'\x0a\x0a\x0a\x0a\x0a')

``fromsize`` will behave just as the current constructors behave when passed a
single integer, while allowing for non-zero fill values when needed.


Addition of explicit "single byte" constructors
-----------------------------------------------

As binary counterparts to the text ``chr`` function, this PEP proposes
the addition of an explicit ``fromint`` alternative constructor as a class
method on both ``bytes`` and ``bytearray``::

    >>> bytes.fromint(65)
    b'A'
    >>> bytearray.fromint(65)
    bytearray(b'A')

These methods will only accept integers in the range 0 to 255 (inclusive)::

    >>> bytes.fromint(512)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    ValueError: integer must be in range(0, 256)

    >>> bytes.fromint(1.0)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: 'float' object cannot be interpreted as an integer

The documentation of the ``ord`` builtin will be updated to explicitly note
that ``bytes.fromint`` is the primary inverse operation for binary data, while
``chr`` is the inverse operation for text data, and that ``bytearray.fromint``
also exists.

Behaviorally, ``bytes.fromint(x)`` will be equivalent to the current
``bytes([x])`` (and similarly for ``bytearray``). The new spelling is
expected to be easier to discover and easier to read (especially when used
in conjunction with indexing operations on binary sequence types).

As a separate method, the new spelling will also work better with higher
order functions like ``map``.

These new methods intentionally do NOT offer the same level of general integer
support as the existing ``int.to_bytes`` conversion method, which allows
arbitrarily large integers to be converted to arbitrarily long bytes objects. The
restriction to only accept positive integers that fit in a single byte means
that no byte order information is needed, and there is no need to handle
negative numbers. The documentation of the new methods will refer readers to
``int.to_bytes`` for use cases where handling of arbitrary integers is needed.


Addition of "ascii" constructors
--------------------------------

In Python 2 converting an object, such as the integer ``123``, to bytes (aka the
Python 2 ``str``) was as simple as::

   >>> str(123)
   '123'
   
With Python 3 that became the more verbose::

   >>> str(123).encode('ascii')

This PEP proposes that an ``ascii`` method be added to ``bytes`` and ``bytearray``
to handle this use-case::

   >>> bytes.ascii(123)
   b'123'


Addition of "getbyte" method to retrieve a single byte
------------------------------------------------------

This PEP proposes that ``bytes`` and ``bytearray`` gain the method ``getbyte``
which will always return ``bytes``::

    >>> b'abc'.getbyte(0)
    b'a'

If an index is asked for that doesn't exist, ``IndexError`` is raised::

    >>> b'abc'.getbyte(9)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    IndexError: index out of range


Addition of optimised iterator methods that produce ``bytes`` objects
---------------------------------------------------------------------

This PEP proposes that ``bytes`` and ``bytearray`` gain an optimised
``iterbytes`` method that produces length 1 ``bytes`` objects rather than
integers::

    for x in data.iterbytes():
        # x is a length 1 ``bytes`` object, rather than an integer

For example::

    >>> tuple(b"ABC".iterbytes())
    (b'A', b'B', b'C')


Design discussion
=================

Why not rely on sequence repetition to create zero-initialised sequences?
-------------------------------------------------------------------------

Zero-initialised sequences can be created via sequence repetition::

    >>> b'\x00' * 3
    b'\x00\x00\x00'
    >>> bytearray(b'\x00') * 3
    bytearray(b'\x00\x00\x00')

However, this was also the case when the ``bytearray`` type was originally
designed, and the decision was made to add explicit support for it in the
type constructor. The immutable ``bytes`` type then inherited that feature
when it was introduced in PEP 3137.

This PEP isn't revisiting that original design decision, just changing the
spelling as users sometimes find the current behavior of the binary sequence
constructors surprising. In particular, there's a reasonable case to be made
that ``bytes(x)`` (where ``x`` is an integer) should behave like the
``bytes.fromint(x)`` proposal in this PEP. Providing both behaviors as separate
class methods avoids that ambiguity.


Omitting the originally proposed builtin function
-------------------------------------------------

When submitted to the Steering Council, this PEP proposed the introduction of
a ``bchr`` builtin (with the same behaviour as ``bytes.fromint``), recreating
the ``ord``/``chr``/``unichr`` trio from Python 2 under a different naming
scheme (``ord``/``bchr``/``chr``).

The SC indicated they didn't think this functionality was needed often enough
to justify offering two ways of doing the same thing, especially when one of
those ways was a new builtin function. That part of the proposal was therefore
dropped as being redundant with the ``bytes.fromint`` alternate constructor.

Developers that use this method frequently will instead have the option to
define their own ``bchr = bytes.fromint`` aliases.


Scope limitation: memoryview
----------------------------

Updating ``memoryview`` with the new item retrieval methods is outside the scope
of this PEP.


References
==========

.. [1] Initial March 2014 discussion thread on python-ideas
   (https://mail.python.org/pipermail/python-ideas/2014-March/027295.html)
.. [2] Guido's initial feedback in that thread
   (https://mail.python.org/pipermail/python-ideas/2014-March/027376.html)
.. [3] Issue proposing moving zero-initialised sequences to a dedicated API
   (http://bugs.python.org/issue20895)
.. [4] Issue proposing to use calloc() for zero-initialised binary sequences
   (http://bugs.python.org/issue21644)
.. [5] August 2014 discussion thread on python-dev
   (https://mail.python.org/pipermail/python-ideas/2014-March/027295.html)
.. [6] June 2016 discussion thread on python-dev
   (https://mail.python.org/pipermail/python-dev/2016-June/144875.html)


Copyright
=========

This document has been placed in the public domain.