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>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 2014-03-30
Python-Version: 3.5
Post-History: 2014-03-30 2014-08-15 2014-08-16


Abstract
========

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.

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

* Deprecate passing single integer values to ``bytes`` and ``bytearray``
* Add ``bytes.zeros`` and ``bytearray.zeros`` alternative constructors
* Add ``bytes.byte`` and ``bytearray.byte`` alternative constructors
* Add ``bytes.iterbytes``, ``bytearray.iterbytes`` and
  ``memoryview.iterbytes`` alternative iterators


Proposals
=========

Deprecation of current "zero-initialised sequence" behaviour
------------------------------------------------------------

Currently, the ``bytes`` and ``bytearray`` constructors accept an integer
argument and interpret it as meaning to create a zero-initialised sequence
of the given size::

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

This PEP proposes to deprecate that behaviour in Python 3.5, and remove it
entirely in Python 3.6.

No other changes are proposed to the existing constructors.


Addition of explicit "zero-initialised sequence" constructors
-------------------------------------------------------------

To replace the deprecated behaviour, this PEP proposes the addition of an
explicit ``zeros`` alternative constructor as a class method on both
``bytes`` and ``bytearray``::

    >>> bytes.zeros(3)
    b'\x00\x00\x00'
    >>> bytearray.zeros(3)
    bytearray(b'\x00\x00\x00')

It will behave just as the current constructors behave when passed a single
integer.

The specific choice of ``zeros`` as the alternative constructor name is taken
from the corresponding initialisation function in NumPy (although, as these
are 1-dimensional sequence types rather than N-dimensional matrices, the
constructors take a length as input rather than a shape tuple)


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

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

    >>> bytes.byte(3)
    b'\x03'
    >>> bytearray.byte(3)
    bytearray(b'\x03')

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

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

    >>> bytes.byte(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.byte`` is the inverse operation for binary data, while ``chr``
is the inverse operation for text data.

Behaviourally, ``bytes.byte(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``.


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

This PEP proposes that ``bytes``, ``bytearray`` and ``memoryview`` 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

The method can be used with arbitrary buffer exporting objects by wrapping
them in a ``memoryview`` instance first::

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

For ``memoryview``, the semantics of ``iterbytes()`` are defined such that::

    memview.tobytes() == b''.join(memview.iterbytes())

This allows the raw bytes of the memory view to be iterated over without
needing to make a copy, regardless of the defined shape and format.

The main advantage this method offers over the ``map(bytes.byte, data)``
approach is that it is guaranteed *not* to fail midstream with a
``ValueError`` or ``TypeError``. By contrast, when using the ``map`` based
approach, the type and value of the individual items in the iterable are
only checked as they are retrieved and passed through the ``bytes.byte``
constructor.


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 behaviour 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.byte(x)`` proposal in this PEP. Providing both behaviours as separate
class methods avoids that ambiguity.


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)


Copyright
=========

This document has been placed in the public domain.


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