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

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

Source code: :source:Lib/xdrlib.py

.. 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 attribute :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 as instance: print('packing the double failed:', instance.msg)