:mod:`xdrlib` --- Encode and decode XDR data

============================================

.. module:: xdrlib

   :synopsis: Encoders and decoders for the External Data Representation (XDR).

.. index::

   single: XDR

   single: External Data Representation

The :mod:`xdrlib` module supports the External Data Representation Standard as

described in :rfc:`1014`, written by Sun Microsystems, Inc. June 1987.  It

supports most of the data types described in the RFC.

The :mod:`xdrlib` module defines two classes, one for packing variables into XDR

representation, and another for unpacking from XDR representation.  There are

also two exception classes.

.. class:: Packer()

   :class:`Packer` is the class for packing data into XDR representation. The

   :class:`Packer` class is instantiated with no arguments.

.. class:: Unpacker(data)

   ``Unpacker`` is the complementary class which unpacks XDR data values from a

   string buffer.  The input buffer is given as *data*.

.. seealso::

   :rfc:`1014` - XDR: External Data Representation Standard

      This RFC defined the encoding of data which was XDR at the time this module was

      originally written.  It has apparently been obsoleted by :rfc:`1832`.

   :rfc:`1832` - XDR: External Data Representation Standard

      Newer RFC that provides a revised definition of XDR.

.. _xdr-packer-objects:

Packer Objects

--------------

:class:`Packer` instances have the following methods:

.. method:: Packer.get_buffer()

   Returns the current pack buffer as a string.

.. method:: Packer.reset()

   Resets the pack buffer to the empty string.

In general, you can pack any of the most common XDR data types by calling the

appropriate ``pack_type()`` method.  Each method takes a single argument, the

value to pack.  The following simple data type packing methods are supported:

:meth:`pack_uint`, :meth:`pack_int`, :meth:`pack_enum`, :meth:`pack_bool`,

:meth:`pack_uhyper`, and :meth:`pack_hyper`.

.. method:: Packer.pack_float(value)

   Packs the single-precision floating point number *value*.

.. method:: Packer.pack_double(value)

   Packs the double-precision floating point number *value*.

The following methods support packing strings, bytes, and opaque data:

.. method:: Packer.pack_fstring(n, s)

   Packs a fixed length string, *s*.  *n* is the length of the string but it is

   *not* packed into the data buffer.  The string is padded with null bytes if

   necessary to guaranteed 4 byte alignment.

.. method:: Packer.pack_fopaque(n, data)

   Packs a fixed length opaque data stream, similarly to :meth:`pack_fstring`.

.. method:: Packer.pack_string(s)

   Packs a variable length string, *s*.  The length of the string is first packed

   as an unsigned integer, then the string data is packed with

   :meth:`pack_fstring`.

.. method:: Packer.pack_opaque(data)

   Packs a variable length opaque data string, similarly to :meth:`pack_string`.

.. method:: Packer.pack_bytes(bytes)

   Packs a variable length byte stream, similarly to :meth:`pack_string`.

The following methods support packing arrays and lists:

.. method:: Packer.pack_list(list, pack_item)

   Packs a *list* of homogeneous items.  This method is useful for lists with an

   indeterminate size; i.e. the size is not available until the entire list has

   been walked.  For each item in the list, an unsigned integer ``1`` is packed

   first, followed by the data value from the list.  *pack_item* is the function

   that is called to pack the individual item.  At the end of the list, an unsigned

   integer ``0`` is packed.

   For example, to pack a list of integers, the code might appear like this::

      import xdrlib

      p = xdrlib.Packer()

      p.pack_list([1, 2, 3], p.pack_int)

.. method:: Packer.pack_farray(n, array, pack_item)

   Packs a fixed length list (*array*) of homogeneous items.  *n* is the length of

   the list; it is *not* packed into the buffer, but a :exc:`ValueError` exception

   is raised if ``len(array)`` is not equal to *n*.  As above, *pack_item* is the

   function used to pack each element.

.. method:: Packer.pack_array(list, pack_item)

   Packs a variable length *list* of homogeneous items.  First, the length of the

   list is packed as an unsigned integer, then each element is packed as in

   :meth:`pack_farray` above.

.. _xdr-unpacker-objects:

Unpacker Objects

----------------

The :class:`Unpacker` class offers the following methods:

.. method:: Unpacker.reset(data)

   Resets the string buffer with the given *data*.

.. method:: Unpacker.get_position()

   Returns the current unpack position in the data buffer.

.. method:: Unpacker.set_position(position)

   Sets the data buffer unpack position to *position*.  You should be careful about

   using :meth:`get_position` and :meth:`set_position`.

.. method:: Unpacker.get_buffer()

   Returns the current unpack data buffer as a string.

.. method:: Unpacker.done()

   Indicates unpack completion.  Raises an :exc:`Error` exception if all of the

   data has not been unpacked.

In addition, every data type that can be packed with a :class:`Packer`, can be

unpacked with an :class:`Unpacker`.  Unpacking methods are of the form

``unpack_type()``, and take no arguments.  They return the unpacked object.

.. method:: Unpacker.unpack_float()

   Unpacks a single-precision floating point number.

.. method:: Unpacker.unpack_double()

   Unpacks a double-precision floating point number, similarly to

   :meth:`unpack_float`.

In addition, the following methods unpack strings, bytes, and opaque data:

.. method:: Unpacker.unpack_fstring(n)

   Unpacks and returns a fixed length string.  *n* is the number of characters

   expected.  Padding with null bytes to guaranteed 4 byte alignment is assumed.

.. method:: Unpacker.unpack_fopaque(n)

   Unpacks and returns a fixed length opaque data stream, similarly to

   :meth:`unpack_fstring`.

.. method:: Unpacker.unpack_string()

   Unpacks and returns a variable length string.  The length of the string is first

   unpacked as an unsigned integer, then the string data is unpacked with

   :meth:`unpack_fstring`.

.. method:: Unpacker.unpack_opaque()

   Unpacks and returns a variable length opaque data string, similarly to

   :meth:`unpack_string`.

.. method:: Unpacker.unpack_bytes()

   Unpacks and returns a variable length byte stream, similarly to

   :meth:`unpack_string`.

The following methods support unpacking arrays and lists:

.. method:: Unpacker.unpack_list(unpack_item)

   Unpacks and returns a list of homogeneous items.  The list is unpacked one

   element at a time by first unpacking an unsigned integer flag.  If the flag is

   ``1``, then the item is unpacked and appended to the list.  A flag of ``0``

   indicates the end of the list.  *unpack_item* is the function that is called to

   unpack the items.

.. method:: Unpacker.unpack_farray(n, unpack_item)

   Unpacks and returns (as a list) a fixed length array of homogeneous items.  *n*

   is number of list elements to expect in the buffer. As above, *unpack_item* is

   the function used to unpack each element.

.. method:: Unpacker.unpack_array(unpack_item)

   Unpacks and returns a variable length *list* of homogeneous items. First, the

   length of the list is unpacked as an unsigned integer, then each element is

   unpacked as in :meth:`unpack_farray` above.

.. _xdr-exceptions:

Exceptions

----------

Exceptions in this module are coded as class instances:

.. exception:: Error

   The base exception class.  :exc:`Error` has a single public data member

   :attr:`msg` containing the description of the error.

.. exception:: ConversionError

   Class derived from :exc:`Error`.  Contains no additional instance variables.

Here is an example of how you would catch one of these exceptions::

   import xdrlib

   p = xdrlib.Packer()

   try:

       p.pack_double(8.01)

   except xdrlib.ConversionError, instance:

       print 'packing the double failed:', instance.msg