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PEP: 358
Title: The "bytes" Object
Version: 5311720232e9
Last-Modified:  2009-01-19 16:08:45 +0000 (Mon, 19 Jan 2009)
Author: Neil Schemenauer <nas at>, Guido van Rossum <guido at>
Status: Final
Type: Standards Track
Content-Type: text/plain
Created: 15-Feb-2006
Python-Version: 2.6, 3.0


    This PEP has partially been superseded by PEP 3137.


    This PEP outlines the introduction of a raw bytes sequence type.
    Adding the bytes type is one step in the transition to Unicode
    based str objects which will be introduced in Python 3.0.

    The PEP describes how the bytes type should work in Python 2.6, as
    well as how it should work in Python 3.0.  (Occasionally there are
    differences because in Python 2.6, we have two string types, str
    and unicode, while in Python 3.0 we will only have one string
    type, whose name will be str but whose semantics will be like the
    2.6 unicode type.)


    Python's current string objects are overloaded.  They serve to hold
    both sequences of characters and sequences of bytes.  This
    overloading of purpose leads to confusion and bugs.  In future
    versions of Python, string objects will be used for holding
    character data.  The bytes object will fulfil the role of a byte
    container.  Eventually the unicode type will be renamed to str
    and the old str type will be removed.


    A bytes object stores a mutable sequence of integers that are in
    the range 0 to 255.  Unlike string objects, indexing a bytes
    object returns an integer.  Assigning or comparing an object that
    is not an integer to an element causes a TypeError exception.
    Assigning an element to a value outside the range 0 to 255 causes
    a ValueError exception.  The .__len__() method of bytes returns
    the number of integers stored in the sequence (i.e. the number of

    The constructor of the bytes object has the following signature:

        bytes([initializer[, encoding]])

    If no arguments are provided then a bytes object containing zero
    elements is created and returned.  The initializer argument can be
    a string (in 2.6, either str or unicode), an iterable of integers,
    or a single integer.  The pseudo-code for the constructor
    (optimized for clear semantics, not for speed) is:

        def bytes(initializer=0, encoding=None):
            if isinstance(initializer, int): # In 2.6, int -> (int, long)
                initializer = [0]*initializer
            elif isinstance(initializer, basestring):
                if isinstance(initializer, unicode): # In 3.0, "if True"
                    if encoding is None:
                        # In 3.0, raise TypeError("explicit encoding required")
                        encoding = sys.getdefaultencoding()
                    initializer = initializer.encode(encoding)
                initializer = [ord(c) for c in initializer]
                if encoding is not None:
                    raise TypeError("no encoding allowed for this initializer")
                tmp = []
                for c in initializer:
                    if not isinstance(c, int):
                        raise TypeError("initializer must be iterable of ints")
                    if not 0 <= c < 256:
                        raise ValueError("initializer element out of range")
                initializer = tmp
            new = <new bytes object of length len(initializer)>
            for i, c in enumerate(initializer):
                new[i] = c
            return new

    The .__repr__() method returns a string that can be evaluated to
    generate a new bytes object containing a bytes literal:

        >>> bytes([10, 20, 30])

    The object has a .decode() method equivalent to the .decode()
    method of the str object.  The object has a classmethod .fromhex()
    that takes a string of characters from the set [0-9a-fA-F ] and
    returns a bytes object (similar to binascii.unhexlify).  For

        >>> bytes.fromhex('5c5350ff')
        >>> bytes.fromhex('5c 53 50 ff')

    The object has a .hex() method that does the reverse conversion
    (similar to binascii.hexlify):

        >> bytes([92, 83, 80, 255]).hex()

    The bytes object has some methods similar to list methods, and
    others similar to str methods.  Here is a complete list of
    methods, with their approximate signatures:

        .__add__(bytes) -> bytes
        .__contains__(int | bytes) -> bool
        .__delitem__(int | slice) -> None
        .__delslice__(int, int) -> None
        .__eq__(bytes) -> bool
        .__ge__(bytes) -> bool
        .__getitem__(int | slice) -> int | bytes
        .__getslice__(int, int) -> bytes
        .__gt__(bytes) -> bool
        .__iadd__(bytes) -> bytes
        .__imul__(int) -> bytes
        .__iter__() -> iterator
        .__le__(bytes) -> bool
        .__len__() -> int
        .__lt__(bytes) -> bool
        .__mul__(int) -> bytes
        .__ne__(bytes) -> bool
        .__reduce__(...) -> ...
        .__reduce_ex__(...) -> ...
        .__repr__() -> str
        .__reversed__() -> bytes
        .__rmul__(int) -> bytes
        .__setitem__(int | slice, int | iterable[int]) -> None
        .__setslice__(int, int, iterable[int]) -> Bote
        .append(int) -> None
        .count(int) -> int
        .decode(str) -> str | unicode # in 3.0, only str
        .endswith(bytes) -> bool
        .extend(iterable[int]) -> None
        .find(bytes) -> int
        .index(bytes | int) -> int
        .insert(int, int) -> None
        .join(iterable[bytes]) -> bytes
        .partition(bytes) -> (bytes, bytes, bytes)
        .pop([int]) -> int
        .remove(int) -> None
        .replace(bytes, bytes) -> bytes
        .rindex(bytes | int) -> int
        .rpartition(bytes) -> (bytes, bytes, bytes)
        .split(bytes) -> list[bytes]
        .startswith(bytes) -> bool
        .reverse() -> None
        .rfind(bytes) -> int
        .rindex(bytes | int) -> int
        .rsplit(bytes) -> list[bytes]
        .translate(bytes, [bytes]) -> bytes

    Note the conspicuous absence of .isupper(), .upper(), and friends.
    (But see "Open Issues" below.)  There is no .__hash__() because
    the object is mutable.  There is no use case for a .sort() method.

    The bytes type also supports the buffer interface, supporting
    reading and writing binary (but not character) data.

Out of Scope Issues

    * Python 3k will have a much different I/O subsystem.  Deciding
      how that I/O subsystem will work and interact with the bytes
      object is out of the scope of this PEP.  The expectation however
      is that binary I/O will read and write bytes, while text I/O
      will read strings.  Since the bytes type supports the buffer
      interface, the existing binary I/O operations in Python 2.6 will
      support bytes objects.

    * It has been suggested that a special method named .__bytes__()
      be added to the language to allow objects to be converted into
      byte arrays.  This decision is out of scope.

    * A bytes literal of the form b"..." is also proposed.  This is
      the subject of PEP 3112.

Open Issues

    * The .decode() method is redundant since a bytes object b can
      also be decoded by calling unicode(b, <encoding>) (in 2.6) or
      str(b, <encoding>) (in 3.0).  Do we need encode/decode methods
      at all?  In a sense the spelling using a constructor is cleaner.

    * Need to specify the methods still more carefully.

    * Pickling and marshalling support need to be specified.

    * Should all those list methods really be implemented?

    * A case could be made for supporting .ljust(), .rjust(),
      .center() with a mandatory second argument.

    * A case could be made for supporting .split() with a mandatory

    * A case could even be made for supporting .islower(), .isupper(),
      .isspace(), .isalpha(), .isalnum(), .isdigit() and the
      corresponding conversions (.lower() etc.), using the ASCII
      definitions for letters, digits and whitespace.  If this is
      accepted, the cases for .ljust(), .rjust(), .center() and
      .split() become much stronger, and they should have default
      arguments as well, using an ASCII space or all ASCII whitespace
      (for .split()).

Frequently Asked Questions

    Q: Why have the optional encoding argument when the encode method of
       Unicode objects does the same thing?

    A: In the current version of Python, the encode method returns a str
       object and we cannot change that without breaking code.  The
       construct bytes(s.encode(...)) is expensive because it has to
       copy the byte sequence multiple times.  Also, Python generally
       provides two ways of converting an object of type A into an
       object of type B: ask an A instance to convert itself to a B, or
       ask the type B to create a new instance from an A. Depending on
       what A and B are, both APIs make sense; sometimes reasons of
       decoupling require that A can't know about B, in which case you
       have to use the latter approach; sometimes B can't know about A,
       in which case you have to use the former.

    Q: Why does bytes ignore the encoding argument if the initializer is
       a str?  (This only applies to 2.6.)

    A: There is no sane meaning that the encoding can have in that case.
       str objects *are* byte arrays and they know nothing about the
       encoding of character data they contain.  We need to assume that
       the programmer has provided a str object that already uses the
       desired encoding. If you need something other than a pure copy of
       the bytes then you need to first decode the string.  For example:

           bytes(s.decode(encoding1), encoding2)

    Q: Why not have the encoding argument default to Latin-1 (or some
       other encoding that covers the entire byte range) rather than

    A: The system default encoding for Python is ASCII.  It seems least
       confusing to use that default.  Also, in Py3k, using Latin-1 as
       the default might not be what users expect.  For example, they
       might prefer a Unicode encoding.  Any default will not always
       work as expected.  At least ASCII will complain loudly if you try
       to encode non-ASCII data.


    This document has been placed in the public domain.