python-peps/peps/pep-3137.rst

PEP: 3137
Title: Immutable Bytes and Mutable Buffer
Version: $Revision$
Last-Modified: $Date$
Author: Guido van Rossum <guido@python.org>
Status: Final
Type: Standards Track
Content-Type: text/x-rst
Created: 26-Sep-2007
Python-Version: 3.0
Post-History: 26-Sep-2007, 30-Sep-2007

Introduction
============

After releasing Python 3.0a1 with a mutable bytes type, pressure
mounted to add a way to represent immutable bytes.  Gregory P. Smith
proposed a patch that would allow making a bytes object temporarily
immutable by requesting that the data be locked using the new buffer
API from :pep:`3118`.  This did not seem the right approach to me.

Jeffrey Yasskin, with the help of Adam Hupp, then prepared a patch to
make the bytes type immutable (by crudely removing all mutating APIs)
and fix the fall-out in the test suite.  This showed that there aren't
all that many places that depend on the mutability of bytes, with the
exception of code that builds up a return value from small pieces.

Thinking through the consequences, and noticing that using the array
module as an ersatz mutable bytes type is far from ideal, and
recalling a proposal put forward earlier by Talin, I floated the
suggestion to have both a mutable and an immutable bytes type.  (This
had been brought up before, but until seeing the evidence of Jeffrey's
patch I wasn't open to the suggestion.)

Moreover, a possible implementation strategy became clear: use the old
PyString implementation, stripped down to remove locale support and
implicit conversions to/from Unicode, for the immutable bytes type,
and keep the new PyBytes implementation as the mutable bytes type.

The ensuing discussion made it clear that the idea is welcome but
needs to be specified more precisely.  Hence this PEP.

Advantages
==========

One advantage of having an immutable bytes type is that code objects
can use these.  It also makes it possible to efficiently create hash
tables using bytes for keys; this may be useful when parsing protocols
like HTTP or SMTP which are based on bytes representing text.

Porting code that manipulates binary data (or encoded text) in Python
2.x will be easier using the new design than using the original 3.0
design with mutable bytes; simply replace ``str`` with ``bytes`` and
change '...' literals into b'...' literals.

Naming
======

I propose the following type names at the Python level:

- ``bytes`` is an immutable array of bytes (PyString)

- ``bytearray`` is a mutable array of bytes (PyBytes)

- ``memoryview`` is a bytes view on another object (PyMemory)

The old type named ``buffer`` is so similar to the new type
``memoryview``, introduce by :pep:`3118`, that it is redundant.  The rest
of this PEP doesn't discuss the functionality of ``memoryview``; it is
just mentioned here to justify getting rid of the old ``buffer`` type.
(An earlier version of this PEP proposed ``buffer`` as the new name
for PyBytes; in the end this name was deemed to confusing given the
many other uses of the word buffer.)

While eventually it makes sense to change the C API names, this PEP
maintains the old C API names, which should be familiar to all.

Summary
-------

Here's a simple ASCII-art table summarizing the type names in various
Python versions::

    +--------------+-------------+------------+--------------------------+
    | C name       | 2.x    repr | 3.0a1 repr | 3.0a2               repr |
    +--------------+-------------+------------+--------------------------+
    | PyUnicode    | unicode u'' | str     '' | str                   '' |
    | PyString     | str      '' | str8   s'' | bytes                b'' |
    | PyBytes      | N/A         | bytes  b'' | bytearray bytearray(b'') |
    | PyBuffer     | buffer      | buffer     | N/A                      |
    | PyMemoryView | N/A         | memoryview | memoryview         <...> |
    +--------------+-------------+------------+--------------------------+

Literal Notations
=================

The b'...' notation introduced in Python 3.0a1 returns an immutable
bytes object, whatever variation is used.  To create a mutable array
of bytes, use bytearray(b'...') or bytearray([...]).  The latter form
takes a list of integers in range(256).

Functionality
=============

PEP 3118 Buffer API
-------------------

Both bytes and bytearray implement the :pep:`3118` buffer API.  The bytes
type only implements read-only requests; the bytearray type allows
writable and data-locked requests as well.  The element data type is
always 'B' (i.e. unsigned byte).

Constructors
------------

There are four forms of constructors, applicable to both bytes and
bytearray:

- ``bytes(<bytes>)``, ``bytes(<bytearray>)``, ``bytearray(<bytes>)``,
  ``bytearray(<bytearray>)``: simple copying constructors, with the
  note that ``bytes(<bytes>)`` might return its (immutable)
  argument, but ``bytearray(<bytearray>)`` always makes a copy.

- ``bytes(<str>, <encoding>[, <errors>])``, ``bytearray(<str>,
  <encoding>[, <errors>])``: encode a text string.  Note that the
  ``str.encode()`` method returns an *immutable* bytes object.  The
  <encoding> argument is mandatory; <errors> is optional.
  <encoding> and <errors>, if given, must be ``str`` instances.

- ``bytes(<memory view>)``, ``bytearray(<memory view>)``: construct
  a bytes or bytearray object from anything that implements the PEP
  3118 buffer API.

- ``bytes(<iterable of ints>)``, ``bytearray(<iterable of ints>)``:
  construct a bytes or bytearray object from a stream of integers in
  range(256).

- ``bytes(<int>)``, ``bytearray(<int>)``: construct a
  zero-initialized bytes or bytearray object of a given length.

Comparisons
-----------

The bytes and bytearray types are comparable with each other and
orderable, so that e.g. b'abc' == bytearray(b'abc') < b'abd'.

Comparing either type to a str object for equality returns False
regardless of the contents of either operand.  Ordering comparisons
with str raise TypeError.  This is all conformant to the standard
rules for comparison and ordering between objects of incompatible
types.

(**Note:** in Python 3.0a1, comparing a bytes instance with a str
instance would raise TypeError, on the premise that this would catch
the occasional mistake quicker, especially in code ported from Python
2.x.  However, a long discussion on the python-3000 list pointed out
so many problems with this that it is clearly a bad idea, to be rolled
back in 3.0a2 regardless of the fate of the rest of this PEP.)

Slicing
-------

Slicing a bytes object returns a bytes object.  Slicing a bytearray
object returns a bytearray object.

Slice assignment to a bytearray object accepts anything that
implements the :pep:`3118` buffer API, or an iterable of integers in
range(256).

Indexing
--------

Indexing bytes and bytearray returns small ints (like the bytes type in
3.0a1, and like lists or array.array('B')).

Assignment to an item of a bytearray object accepts an int in
range(256).  (To assign from a bytes sequence, use a slice
assignment.)

Str() and Repr()
----------------

The str() and repr() functions return the same thing for these
objects.  The repr() of a bytes object returns a b'...' style literal.
The repr() of a bytearray returns a string of the form "bytearray(b'...')".

Operators
---------

The following operators are implemented by the bytes and bytearray
types, except where mentioned:

- ``b1 + b2``: concatenation.  With mixed bytes/bytearray operands,
  the return type is that of the first argument (this seems arbitrary
  until you consider how ``+=`` works).

- ``b1 += b2``: mutates b1 if it is a bytearray object.

- ``b * n``, ``n * b``: repetition; n must be an integer.

- ``b *= n``: mutates b if it is a bytearray object.

- ``b1 in b2``, ``b1 not in b2``: substring test; b1 can be any
  object implementing the :pep:`3118` buffer API.

- ``i in b``, ``i not in b``: single-byte membership test; i must
  be an integer (if it is a length-1 bytes array, it is considered
  to be a substring test, with the same outcome).

- ``len(b)``: the number of bytes.

- ``hash(b)``: the hash value; only implemented by the bytes type.

Note that the % operator is *not* implemented.  It does not appear
worth the complexity.

Methods
-------

The following methods are implemented by bytes as well as bytearray, with
similar semantics.  They accept anything that implements the :pep:`3118`
buffer API for bytes arguments, and return the same type as the object
whose method is called ("self")::

  .capitalize(), .center(), .count(), .decode(), .endswith(),
  .expandtabs(), .find(), .index(), .isalnum(), .isalpha(), .isdigit(),
  .islower(), .isspace(), .istitle(), .isupper(), .join(), .ljust(),
  .lower(), .lstrip(), .partition(), .replace(), .rfind(), .rindex(),
  .rjust(), .rpartition(), .rsplit(), .rstrip(), .split(),
  .splitlines(), .startswith(), .strip(), .swapcase(), .title(),
  .translate(), .upper(), .zfill()

This is exactly the set of methods present on the str type in Python
2.x, with the exclusion of .encode().  The signatures and semantics
are the same too.  However, whenever character classes like letter,
whitespace, lower case are used, the ASCII definitions of these
classes are used.  (The Python 2.x str type uses the definitions from
the current locale, settable through the locale module.)  The
.encode() method is left out because of the more strict definitions of
encoding and decoding in Python 3000: encoding always takes a Unicode
string and returns a bytes sequence, and decoding always takes a bytes
sequence and returns a Unicode string.

In addition, both types implement the class method ``.fromhex()``,
which constructs an object from a string containing hexadecimal values
(with or without spaces between the bytes).

The bytearray type implements these additional methods from the
MutableSequence ABC (see :pep:`3119`):

  .extend(), .insert(), .append(), .reverse(), .pop(), .remove().

Bytes and the Str Type
----------------------

Like the bytes type in Python 3.0a1, and unlike the relationship
between str and unicode in Python 2.x, attempts to mix bytes (or
bytearray) objects and str objects without specifying an encoding will
raise a TypeError exception.  (However, comparing bytes/bytearray and
str objects for equality will simply return False; see the section on
Comparisons above.)

Conversions between bytes or bytearray objects and str objects must
always be explicit, using an encoding.  There are two equivalent APIs:
``str(b, <encoding>[, <errors>])`` is equivalent to
``b.decode(<encoding>[, <errors>])``, and
``bytes(s, <encoding>[, <errors>])`` is equivalent to
``s.encode(<encoding>[, <errors>])``.

There is one exception: we can convert from bytes (or bytearray) to str
without specifying an encoding by writing ``str(b)``.  This produces
the same result as ``repr(b)``.  This exception is necessary because
of the general promise that *any* object can be printed, and printing
is just a special case of conversion to str.  There is however no
promise that printing a bytes object interprets the individual bytes
as characters (unlike in Python 2.x).

The str type currently implements the :pep:`3118` buffer API.  While this
is perhaps occasionally convenient, it is also potentially confusing,
because the bytes accessed via the buffer API represent a
platform-depending encoding: depending on the platform byte order and
a compile-time configuration option, the encoding could be UTF-16-BE,
UTF-16-LE, UTF-32-BE, or UTF-32-LE.  Worse, a different implementation
of the str type might completely change the bytes representation,
e.g. to UTF-8, or even make it impossible to access the data as a
contiguous array of bytes at all.  Therefore, the :pep:`3118` buffer API
will be removed from the str type.

The ``basestring`` Type
-----------------------

The ``basestring`` type will be removed from the language.  Code that
used to say ``isinstance(x, basestring)`` should be changed to use
``isinstance(x, str)`` instead.

Pickling
--------

Left as an exercise for the reader.

Copyright
=========

This document has been placed in the public domain.


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