PEP 3112 – Bytes literals in Python 3000
- Author:
- Jason Orendorff <jason.orendorff at gmail.com>
- Status:
- Final
- Type:
- Standards Track
- Requires:
- 358
- Created:
- 23-Feb-2007
- Python-Version:
- 3.0
- Post-History:
- 23-Feb-2007
Table of Contents
Abstract
This PEP proposes a literal syntax for the bytes
objects
introduced in PEP 358. The purpose is to provide a convenient way to
spell ASCII strings and arbitrary binary data.
Motivation
Existing spellings of an ASCII string in Python 3000 include:
bytes('Hello world', 'ascii')
'Hello world'.encode('ascii')
The proposed syntax is:
b'Hello world'
Existing spellings of an 8-bit binary sequence in Python 3000 include:
bytes([0x7f, 0x45, 0x4c, 0x46, 0x01, 0x01, 0x01, 0x00])
bytes('\x7fELF\x01\x01\x01\0', 'latin-1')
'7f454c4601010100'.decode('hex')
The proposed syntax is:
b'\x7f\x45\x4c\x46\x01\x01\x01\x00'
b'\x7fELF\x01\x01\x01\0'
In both cases, the advantages of the new syntax are brevity, some small efficiency gain, and the detection of encoding errors at compile time rather than at runtime. The brevity benefit is especially felt when using the string-like methods of bytes objects:
lines = bdata.split(bytes('\n', 'ascii')) # existing syntax
lines = bdata.split(b'\n') # proposed syntax
And when converting code from Python 2.x to Python 3000:
sok.send('EXIT\r\n') # Python 2.x
sok.send('EXIT\r\n'.encode('ascii')) # Python 3000 existing
sok.send(b'EXIT\r\n') # proposed
Grammar Changes
The proposed syntax is an extension of the existing string syntax [1].
The new syntax for strings, including the new bytes literal, is:
stringliteral: [stringprefix] (shortstring | longstring)
stringprefix: "b" | "r" | "br" | "B" | "R" | "BR" | "Br" | "bR"
shortstring: "'" shortstringitem* "'" | '"' shortstringitem* '"'
longstring: "'''" longstringitem* "'''" | '"""' longstringitem* '"""'
shortstringitem: shortstringchar | escapeseq
longstringitem: longstringchar | escapeseq
shortstringchar:
<any source character except "\" or newline or the quote>
longstringchar: <any source character except "\">
escapeseq: "\" NL
| "\\" | "\'" | '\"'
| "\a" | "\b" | "\f" | "\n" | "\r" | "\t" | "\v"
| "\ooo" | "\xhh"
| "\uxxxx" | "\Uxxxxxxxx" | "\N{name}"
The following additional restrictions apply only to bytes literals
(stringliteral
tokens with b
or B
in the
stringprefix
):
- Each
shortstringchar
orlongstringchar
must be a character between 1 and 127 inclusive, regardless of any encoding declaration [2] in the source file. - The Unicode-specific escape sequences
\u
xxxx,\U
xxxxxxxx, and\N{
name}
are unrecognized in Python 2.x and forbidden in Python 3000.
Adjacent bytes literals are subject to the same concatenation rules as adjacent string literals [3]. A bytes literal adjacent to a string literal is an error.
Semantics
Each evaluation of a bytes literal produces a new bytes
object.
The bytes in the new object are the bytes represented by the
shortstringitem
or longstringitem
parts of the literal, in the
same order.
Rationale
The proposed syntax provides a cleaner migration path from Python 2.x
to Python 3000 for most code involving 8-bit strings. Preserving the
old 8-bit meaning of a string literal is usually as simple as adding a
b
prefix. The one exception is Python 2.x strings containing
bytes >127, which must be rewritten using escape sequences.
Transcoding a source file from one encoding to another, and fixing up
the encoding declaration, should preserve the meaning of the program.
Python 2.x non-Unicode strings violate this principle; Python 3000
bytes literals shouldn’t.
A string literal with a b
in the prefix is always a syntax error
in Python 2.5, so this syntax can be introduced in Python 2.6, along
with the bytes
type.
A bytes literal produces a new object each time it is evaluated, like list displays and unlike string literals. This is necessary because bytes literals, like lists and unlike strings, are mutable [4].
Reference Implementation
Thomas Wouters has checked an implementation into the Py3K branch, r53872.
References
Copyright
This document has been placed in the public domain.
Source: https://github.com/python/peps/blob/main/peps/pep-3112.rst
Last modified: 2023-09-09 17:39:29 GMT