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PEP 3112 -- Bytes literals in Python 3000

Title:Bytes literals in Python 3000
Author:Jason Orendorff <jason.orendorff at>
Type:Standards Track


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.


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')

The proposed syntax is:


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
  <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 or longstringchar must be a character between 1 and 127 inclusive, regardless of any encoding declaration [2] in the source file.
  • The Unicode-specific escape sequences \uxxxx, \Uxxxxxxxx, 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.


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.


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.