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