PEP 467 – Minor API improvements for binary sequences
- Nick Coghlan <ncoghlan at gmail.com>, Ethan Furman <ethan at stoneleaf.us>
- Standards Track
- 30-Mar-2014, 15-Aug-2014, 16-Aug-2014, 07-Jun-2016, 01-Sep-2016, 13-Apr-2021, 03-Nov-2021
Table of Contents
- Design discussion
This PEP proposes five small adjustments to the APIs of the
bytearray types to make it easier to operate entirely in the binary domain:
getbytebyte retrieval method
During the initial development of the Python 3 language specification, the
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.
With Python 3 and the split between
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
HTTP communications, among many others.
Addition of explicit “count and byte initialised sequence” constructors
To replace the now discouraged behavior, this PEP proposes the addition of an
fromsize alternative constructor as a class method on both
bytearray whose first argument is the count, and whose
second argument is the fill byte to use (defaults to
>>> 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.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
bytes.fromint is the primary inverse operation for binary data, while
chr is the inverse operation for text data, and that
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
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
str) was as simple as:
>>> str(123) '123'
With Python 3 that became the more verbose:
>>> b'%d' % 123
This PEP proposes that an
ascii method be added to
to handle this use-case:
>>> bytes.ascii(123) b'123'
bytes.ascii() would handle simple ascii-encodable text correctly,
>>> ascii("hello").encode('ascii') b"'hello'"
Addition of “getbyte” method to retrieve a single byte
This PEP proposes that
bytearray gain the method
which will always return
>>> 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
This PEP proposes that
bytearray gain an optimised
iterbytes method that produces length 1
bytes objects rather than
for x in data.iterbytes(): # x is a length 1 ``bytes`` object, rather than an integer
>>> tuple(b"ABC".iterbytes()) (b'A', b'B', b'C')
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
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
bchr builtin (with the same behaviour as
unichr trio from Python 2 under a different naming
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
memoryview with the new item retrieval methods is outside the scope
of this PEP.
- Initial March 2014 discussion thread on python-ideas
- Guido’s initial feedback in that thread
- Issue proposing moving zero-initialised sequences to a dedicated API
- Issue proposing to use calloc() for zero-initialised binary sequences
- August 2014 discussion thread on python-dev
- June 2016 discussion thread on python-dev
This document has been placed in the public domain.
Last modified: 2022-08-17 05:25:53 GMT