python-peps/peps/pep-0296.rst

PEP: 296
Title: Adding a bytes Object Type
Author: Scott Gilbert <xscottg at yahoo.com>
Status: Withdrawn
Type: Standards Track
Content-Type: text/x-rst
Created: 12-Jul-2002
Python-Version: 2.3
Post-History:


Notice
=======

This PEP is withdrawn by the author (in favor of :pep:`358`).


Abstract
========

This PEP proposes the creation of a new standard type and builtin
constructor called 'bytes'.  The bytes object is an efficiently
stored array of bytes with some additional characteristics that
set it apart from several implementations that are similar.


Rationale
=========

Python currently has many objects that implement something akin to
the bytes object of this proposal.  For instance the standard
string, buffer, array, and mmap objects are all very similar in
some regards to the bytes object.  Additionally, several
significant third party extensions have created similar objects to
try and fill similar needs.  Frustratingly, each of these objects
is too narrow in scope and is missing critical features to make it
applicable to a wider category of problems.


Specification
=============

The bytes object has the following important characteristics:

1. Efficient underlying array storage via the standard C type "unsigned
   char".  This allows fine grain control over how much memory is
   allocated.  With the alignment restrictions designated in the next
   item, it is trivial for low level extensions to cast the pointer
   to a different type as needed.

   Also, since the object is implemented as an array of bytes, it is
   possible to pass the bytes object to the extensive library of
   routines already in the standard library that presently work with
   strings.  For instance, the bytes object in conjunction with the
   struct module could be used to provide a complete replacement for
   the array module using only Python script.

   If an unusual platform comes to light, one where there isn't a
   native unsigned 8 bit type, the object will do its best to
   represent itself at the Python script level as though it were an
   array of 8 bit unsigned values.  It is doubtful whether many
   extensions would handle this correctly, but Python script could be
   portable in these cases.

2. Alignment of the allocated byte array is whatever is promised by the
   platform implementation of malloc.  A bytes object created from an
   extension can be supplied that provides any arbitrary alignment as
   the extension author sees fit.

   This alignment restriction should allow the bytes object to be
   used as storage for all standard C types - including ``PyComplex``
   objects or other structs of standard C type types.  Further
   alignment restrictions can be provided by extensions as necessary.

3. The bytes object implements a subset of the sequence operations
   provided by string/array objects, but with slightly different
   semantics in some cases.  In particular, a slice always returns a
   new bytes object, but the underlying memory is shared between the
   two objects.  This type of slice behavior has been called creating
   a "view".  Additionally, repetition and concatenation are
   undefined for bytes objects and will raise an exception.

   As these objects are likely to find use in high performance
   applications, one motivation for the decision to use view slicing
   is that copying between bytes objects should be very efficient and
   not require the creation of temporary objects.  The following code
   illustrates this::

       # create two 10 Meg bytes objects
       b1 = bytes(10000000)
       b2 = bytes(10000000)

       # copy from part of one to another with out creating a 1 Meg temporary
       b1[2000000:3000000] = b2[4000000:5000000]

   Slice assignment where the rvalue is not the same length as the
   lvalue will raise an exception.  However, slice assignment will
   work correctly with overlapping slices (typically implemented with
   memmove).

4. The bytes object will be recognized as a native type by the ``pickle`` and
   ``cPickle`` modules for efficient serialization.  (In truth, this is
   the only requirement that can't be implemented via a third party
   extension.)

   Partial solutions to address the need to serialize the data stored
   in a bytes-like object without creating a temporary copy of the
   data into a string have been implemented in the past.  The tofile
   and fromfile methods of the array object are good examples of
   this.  The bytes object will support these methods too.  However,
   pickling is useful in other situations - such as in the shelve
   module, or implementing RPC of Python objects, and requiring the
   end user to use two different serialization mechanisms to get an
   efficient transfer of data is undesirable.

   XXX: Will try to implement pickling of the new bytes object in
   such a way that previous versions of Python will unpickle it as a
   string object.

   When unpickling, the bytes object will be created from memory
   allocated from Python (via ``malloc``).  As such, it will lose any
   additional properties that an extension supplied pointer might
   have provided (special alignment, or special types of memory).

   XXX: Will try to make it so that C subclasses of bytes type can
   supply the memory that will be unpickled into.  For instance, a
   derived class called PageAlignedBytes would unpickle to memory
   that is also page aligned.

   On any platform where an int is 32 bits (most of them), it is
   currently impossible to create a string with a length larger than
   can be represented in 31 bits.  As such, pickling to a string will
   raise an exception when the operation is not possible.

   At least on platforms supporting large files (many of them),
   pickling large bytes objects to files should be possible via
   repeated calls to the ``file.write()`` method.

5. The bytes type supports the ``PyBufferProcs`` interface, but a bytes object
   provides the additional guarantee that the pointer will not be
   deallocated or reallocated as long as a reference to the bytes
   object is held.  This implies that a bytes object is not resizable
   once it is created, but allows the global interpreter lock (GIL)
   to be released while a separate thread manipulates the memory
   pointed to if the ``PyBytes_Check(...)`` test passes.

   This characteristic of the bytes object allows it to be used in
   situations such as asynchronous file I/O or on multiprocessor
   machines where the pointer obtained by ``PyBufferProcs`` will be used
   independently of the global interpreter lock.

   Knowing that the pointer can not be reallocated or freed after the
   GIL is released gives extension authors the capability to get true
   concurrency and make use of additional processors for long running
   computations on the pointer.

6. In C/C++ extensions, the bytes object can be created from a supplied
   pointer and destructor function to free the memory when the
   reference count goes to zero.

   The special implementation of slicing for the bytes object allows
   multiple bytes objects to refer to the same pointer/destructor.
   As such, a refcount will be kept on the actual
   pointer/destructor.  This refcount is separate from the refcount
   typically associated with Python objects.

   XXX: It may be desirable to expose the inner refcounted object as an
   actual Python object.  If a good use case arises, it should be possible
   for this to be implemented later with no loss to backwards compatibility.

7. It is also possible to signify the bytes object as readonly, in this
   case it isn't actually mutable, but does provide the other features of a
   bytes object.

8. The bytes object keeps track of the length of its data with a Python
   ``LONG_LONG`` type.  Even though the current definition for ``PyBufferProcs``
   restricts the length to be the size of an int, this PEP does not propose
   to make any changes there.  Instead, extensions can work around this limit
   by making an explicit ``PyBytes_Check(...)`` call, and if that succeeds they
   can make a ``PyBytes_GetReadBuffer(...)`` or ``PyBytes_GetWriteBuffer``
   call to get the pointer and full length of the object as a ``LONG_LONG``.

   The bytes object will raise an exception if the standard ``PyBufferProcs``
   mechanism is used and the size of the bytes object is greater than can be
   represented by an integer.

   From Python scripting, the bytes object will be subscriptable with longs
   so the 32 bit int limit can be avoided.

   There is still a problem with the ``len()`` function as it is
   ``PyObject_Size()`` and this returns an int as well.  As a workaround,
   the bytes object will provide a ``.length()`` method that will return a long.

9. The bytes object can be constructed at the Python scripting level by
   passing an int/long to the bytes constructor with the number of bytes to
   allocate.  For example::

       b = bytes(100000) # alloc 100K bytes

   The constructor can also take another bytes object.  This will be useful
   for the implementation of unpickling, and in converting a read-write bytes
   object into a read-only one.  An optional second argument will be used to
   designate creation of a readonly bytes object.

10. From the C API, the bytes object can be allocated using any of the
    following signatures::

        PyObject* PyBytes_FromLength(LONG_LONG len, int readonly);
        PyObject* PyBytes_FromPointer(void* ptr, LONG_LONG len, int readonly
                 void (*dest)(void *ptr, void *user), void* user);

    In the ``PyBytes_FromPointer(...)`` function, if the dest function pointer
    is passed in as ``NULL``, it will not be called.  This should only be used
    for creating bytes objects from statically allocated space.

    The user pointer has been called a closure in other places.  It is a
    pointer that the user can use for whatever purposes.  It will be passed to
    the destructor function on cleanup and can be useful for a number of
    things.  If the user pointer is not needed, ``NULL`` should be passed
    instead.

11. The bytes type will be a new style class as that seems to be where all
    standard Python types are headed.


Contrast to existing types
==========================

The most common way to work around the lack of a bytes object has been to
simply use a string object in its place.  Binary files, the struct/array
modules, and several other examples exist of this.  Putting aside the
style issue that these uses typically have nothing to do with text
strings, there is the real problem that strings are not mutable, so direct
manipulation of the data returned in these cases is not possible.  Also,
numerous optimizations in the string module (such as caching the hash
value or interning the pointers) mean that extension authors are on very
thin ice if they try to break the rules with the string object.

The buffer object seems like it was intended to address the purpose that
the bytes object is trying fulfill, but several shortcomings in its
implementation [1]_ have made it less useful in many common cases.  The
buffer object made a different choice for its slicing behavior (it returns
new strings instead of buffers for slicing and other operations), and it
doesn't make many of the promises on alignment or being able to release
the GIL that the bytes object does.

Also in regards to the buffer object, it is not possible to simply replace
the buffer object with the bytes object and maintain backwards
compatibility.  The buffer object provides a mechanism to take the
``PyBufferProcs`` supplied pointer of another object and present it as its
own.  Since the behavior of the other object can not be guaranteed to
follow the same set of strict rules that a bytes object does, it can't be
used in places that a bytes object could.

The array module supports the creation of an array of bytes, but it does
not provide a C API for supplying pointers and destructors to extension
supplied memory.  This makes it unusable for constructing objects out of
shared memory, or memory that has special alignment or locking for things
like DMA transfers.  Also, the array object does not currently pickle.
Finally since the array object allows its contents to grow, via the extend
method, the pointer can be changed if the GIL is not held while using it.

Creating a buffer object from an array object has the same problem of
leaving an invalid pointer when the array object is resized.

The mmap object caters to its particular niche, but does not attempt to
solve a wider class of problems.

Finally, any third party extension can not implement pickling without
creating a temporary object of a standard Python type.  For example, in the
Numeric community, it is unpleasant that a large array can't pickle
without creating a large binary string to duplicate the array data.


Backward Compatibility
======================

The only possibility for backwards compatibility problems that the author
is aware of are in previous versions of Python that try to unpickle data
containing the new bytes type.


Reference Implementation
========================

XXX: Actual implementation is in progress, but changes are still possible
as this PEP gets further review.

The following new files will be added to the Python baseline::

    Include/bytesobject.h  # C interface
    Objects/bytesobject.c  # C implementation
    Lib/test/test_bytes.py # unit testing
    Doc/lib/libbytes.tex   # documentation

The following files will also be modified::

    Include/Python.h       # adding bytesmodule.h include file
    Python/bltinmodule.c   # adding the bytes type object
    Modules/cPickle.c      # adding bytes to the standard types
    Lib/pickle.py          # adding bytes to the standard types

It is possible that several other modules could be cleaned up and
implemented in terms of the bytes object.  The mmap module comes to mind
first, but as noted above it would be possible to reimplement the array
module as a pure Python module.  While it is attractive that this PEP
could actually reduce the amount of source code by some amount, the author
feels that this could cause unnecessary risk for breaking existing
applications and should be avoided at this time.


Additional Notes/Comments
=========================

- Guido van Rossum wondered whether it would make sense to be able
  to create a bytes object from a mmap object.  The mmap object
  appears to support the requirements necessary to provide memory
  for a bytes object.  (It doesn't resize, and the pointer is valid
  for the lifetime of the object.)  As such, a method could be added
  to the mmap module such that a bytes object could be created
  directly from a mmap object.  An initial stab at how this would be
  implemented would be to use the ``PyBytes_FromPointer()`` function
  described above and pass the ``mmap_object`` as the user pointer.  The
  destructor function would decref the ``mmap_object`` for cleanup.

- Todd Miller notes that it may be useful to have two new functions:
  ``PyObject_AsLargeReadBuffer()`` and ``PyObject_AsLargeWriteBuffer`` that are
  similar to ``PyObject_AsReadBuffer()`` and ``PyObject_AsWriteBuffer()``, but
  support getting a ``LONG_LONG`` length in addition to the ``void*`` pointer.
  These functions would allow extension authors to work transparently with
  bytes object (that support ``LONG_LONG`` lengths) and most other buffer like
  objects (which only support int lengths).  These functions could be in
  lieu of, or in addition to, creating a specific ``PyByte_GetReadBuffer()`` and
  ``PyBytes_GetWriteBuffer()`` functions.

  XXX: The author thinks this is very a good idea as it paves the way for
  other objects to eventually support large (64 bit) pointers, and it should
  only affect abstract.c and abstract.h.  Should this be added above?

- It was generally agreed that abusing the segment count of the
  ``PyBufferProcs`` interface is not a good hack to work around the 31 bit
  limitation of the length.  If you don't know what this means, then you're
  in good company.  Most code in the Python baseline, and presumably in many
  third party extensions, punt when the segment count is not 1.


References
==========

.. [1] The buffer interface
       https://mail.python.org/pipermail/python-dev/2000-October/009974.html


Copyright
=========

This document has been placed in the public domain.