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PEP 467 -- Minor API improvements for binary sequences

PEP:467
Title:Minor API improvements for binary sequences
Author:Nick Coghlan <ncoghlan at gmail.com>, Ethan Furman <ethan at stoneleaf.us>
Status:Draft
Type:Standards Track
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 2021-11-03

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:

>>> b'%d' % 123

or even:

>>> 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'

Note that bytes.ascii() would handle simple ascii-encodable text correctly, unlike the ascii()` built-in:

>>> ascii("hello").encode('ascii')
b"'hello'"

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)
Source: https://github.com/python/peps/blob/master/pep-0467.txt