
``xdrlib`` --- Encode and decode XDR data
*****************************************

**Source code:** Lib/xdrlib.py

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

The ``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 ``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 class xdrlib.Packer

   ``Packer`` is the class for packing data into XDR representation.
   The ``Packer`` class is instantiated with no arguments.

class class xdrlib.Unpacker(data)

   ``Unpacker`` is the complementary class which unpacks XDR data
   values from a string buffer.  The input buffer is given as *data*.

See also:

   **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.


Packer Objects
==============

``Packer`` instances have the following methods:

Packer.get_buffer()

   Returns the current pack buffer as a string.

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: ``pack_uint()``, ``pack_int()``,
``pack_enum()``, ``pack_bool()``, ``pack_uhyper()``, and
``pack_hyper()``.

Packer.pack_float(value)

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

Packer.pack_double(value)

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

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

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.

Packer.pack_fopaque(n, data)

   Packs a fixed length opaque data stream, similarly to
   ``pack_fstring()``.

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 ``pack_fstring()``.

Packer.pack_opaque(data)

   Packs a variable length opaque data string, similarly to
   ``pack_string()``.

Packer.pack_bytes(bytes)

   Packs a variable length byte stream, similarly to
   ``pack_string()``.

The following methods support packing arrays and lists:

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)

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
   ``ValueError`` exception is raised if ``len(array)`` is not equal
   to *n*.  As above, *pack_item* is the function used to pack each
   element.

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 ``pack_farray()`` above.


Unpacker Objects
================

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

Unpacker.reset(data)

   Resets the string buffer with the given *data*.

Unpacker.get_position()

   Returns the current unpack position in the data buffer.

Unpacker.set_position(position)

   Sets the data buffer unpack position to *position*.  You should be
   careful about using ``get_position()`` and ``set_position()``.

Unpacker.get_buffer()

   Returns the current unpack data buffer as a string.

Unpacker.done()

   Indicates unpack completion.  Raises an ``Error`` exception if all
   of the data has not been unpacked.

In addition, every data type that can be packed with a ``Packer``, can
be unpacked with an ``Unpacker``.  Unpacking methods are of the form
``unpack_type()``, and take no arguments.  They return the unpacked
object.

Unpacker.unpack_float()

   Unpacks a single-precision floating point number.

Unpacker.unpack_double()

   Unpacks a double-precision floating point number, similarly to
   ``unpack_float()``.

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

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.

Unpacker.unpack_fopaque(n)

   Unpacks and returns a fixed length opaque data stream, similarly to
   ``unpack_fstring()``.

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 ``unpack_fstring()``.

Unpacker.unpack_opaque()

   Unpacks and returns a variable length opaque data string, similarly
   to ``unpack_string()``.

Unpacker.unpack_bytes()

   Unpacks and returns a variable length byte stream, similarly to
   ``unpack_string()``.

The following methods support unpacking arrays and lists:

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.

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.

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 ``unpack_farray()`` above.


Exceptions
==========

Exceptions in this module are coded as class instances:

exception exception xdrlib.Error

   The base exception class.  ``Error`` has a single public data
   member ``msg`` containing the description of the error.

exception exception xdrlib.ConversionError

   Class derived from ``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 as instance:
       print('packing the double failed:', instance.msg)
