1:mod:`xdrlib` --- Encode and decode XDR data
2============================================
3
4.. module:: xdrlib
5   :synopsis: Encoders and decoders for the External Data Representation (XDR).
6
7**Source code:** :source:`Lib/xdrlib.py`
8
9.. index::
10   single: XDR
11   single: External Data Representation
12
13--------------
14
15The :mod:`xdrlib` module supports the External Data Representation Standard as
16described in :rfc:`1014`, written by Sun Microsystems, Inc. June 1987.  It
17supports most of the data types described in the RFC.
18
19The :mod:`xdrlib` module defines two classes, one for packing variables into XDR
20representation, and another for unpacking from XDR representation.  There are
21also two exception classes.
22
23
24.. class:: Packer()
25
26   :class:`Packer` is the class for packing data into XDR representation. The
27   :class:`Packer` class is instantiated with no arguments.
28
29
30.. class:: Unpacker(data)
31
32   ``Unpacker`` is the complementary class which unpacks XDR data values from a
33   string buffer.  The input buffer is given as *data*.
34
35
36.. seealso::
37
38   :rfc:`1014` - XDR: External Data Representation Standard
39      This RFC defined the encoding of data which was XDR at the time this module was
40      originally written.  It has apparently been obsoleted by :rfc:`1832`.
41
42   :rfc:`1832` - XDR: External Data Representation Standard
43      Newer RFC that provides a revised definition of XDR.
44
45
46.. _xdr-packer-objects:
47
48Packer Objects
49--------------
50
51:class:`Packer` instances have the following methods:
52
53
54.. method:: Packer.get_buffer()
55
56   Returns the current pack buffer as a string.
57
58
59.. method:: Packer.reset()
60
61   Resets the pack buffer to the empty string.
62
63In general, you can pack any of the most common XDR data types by calling the
64appropriate ``pack_type()`` method.  Each method takes a single argument, the
65value to pack.  The following simple data type packing methods are supported:
66:meth:`pack_uint`, :meth:`pack_int`, :meth:`pack_enum`, :meth:`pack_bool`,
67:meth:`pack_uhyper`, and :meth:`pack_hyper`.
68
69
70.. method:: Packer.pack_float(value)
71
72   Packs the single-precision floating point number *value*.
73
74
75.. method:: Packer.pack_double(value)
76
77   Packs the double-precision floating point number *value*.
78
79The following methods support packing strings, bytes, and opaque data:
80
81
82.. method:: Packer.pack_fstring(n, s)
83
84   Packs a fixed length string, *s*.  *n* is the length of the string but it is
85   *not* packed into the data buffer.  The string is padded with null bytes if
86   necessary to guaranteed 4 byte alignment.
87
88
89.. method:: Packer.pack_fopaque(n, data)
90
91   Packs a fixed length opaque data stream, similarly to :meth:`pack_fstring`.
92
93
94.. method:: Packer.pack_string(s)
95
96   Packs a variable length string, *s*.  The length of the string is first packed
97   as an unsigned integer, then the string data is packed with
98   :meth:`pack_fstring`.
99
100
101.. method:: Packer.pack_opaque(data)
102
103   Packs a variable length opaque data string, similarly to :meth:`pack_string`.
104
105
106.. method:: Packer.pack_bytes(bytes)
107
108   Packs a variable length byte stream, similarly to :meth:`pack_string`.
109
110The following methods support packing arrays and lists:
111
112
113.. method:: Packer.pack_list(list, pack_item)
114
115   Packs a *list* of homogeneous items.  This method is useful for lists with an
116   indeterminate size; i.e. the size is not available until the entire list has
117   been walked.  For each item in the list, an unsigned integer ``1`` is packed
118   first, followed by the data value from the list.  *pack_item* is the function
119   that is called to pack the individual item.  At the end of the list, an unsigned
120   integer ``0`` is packed.
121
122   For example, to pack a list of integers, the code might appear like this::
123
124      import xdrlib
125      p = xdrlib.Packer()
126      p.pack_list([1, 2, 3], p.pack_int)
127
128
129.. method:: Packer.pack_farray(n, array, pack_item)
130
131   Packs a fixed length list (*array*) of homogeneous items.  *n* is the length of
132   the list; it is *not* packed into the buffer, but a :exc:`ValueError` exception
133   is raised if ``len(array)`` is not equal to *n*.  As above, *pack_item* is the
134   function used to pack each element.
135
136
137.. method:: Packer.pack_array(list, pack_item)
138
139   Packs a variable length *list* of homogeneous items.  First, the length of the
140   list is packed as an unsigned integer, then each element is packed as in
141   :meth:`pack_farray` above.
142
143
144.. _xdr-unpacker-objects:
145
146Unpacker Objects
147----------------
148
149The :class:`Unpacker` class offers the following methods:
150
151
152.. method:: Unpacker.reset(data)
153
154   Resets the string buffer with the given *data*.
155
156
157.. method:: Unpacker.get_position()
158
159   Returns the current unpack position in the data buffer.
160
161
162.. method:: Unpacker.set_position(position)
163
164   Sets the data buffer unpack position to *position*.  You should be careful about
165   using :meth:`get_position` and :meth:`set_position`.
166
167
168.. method:: Unpacker.get_buffer()
169
170   Returns the current unpack data buffer as a string.
171
172
173.. method:: Unpacker.done()
174
175   Indicates unpack completion.  Raises an :exc:`Error` exception if all of the
176   data has not been unpacked.
177
178In addition, every data type that can be packed with a :class:`Packer`, can be
179unpacked with an :class:`Unpacker`.  Unpacking methods are of the form
180``unpack_type()``, and take no arguments.  They return the unpacked object.
181
182
183.. method:: Unpacker.unpack_float()
184
185   Unpacks a single-precision floating point number.
186
187
188.. method:: Unpacker.unpack_double()
189
190   Unpacks a double-precision floating point number, similarly to
191   :meth:`unpack_float`.
192
193In addition, the following methods unpack strings, bytes, and opaque data:
194
195
196.. method:: Unpacker.unpack_fstring(n)
197
198   Unpacks and returns a fixed length string.  *n* is the number of characters
199   expected.  Padding with null bytes to guaranteed 4 byte alignment is assumed.
200
201
202.. method:: Unpacker.unpack_fopaque(n)
203
204   Unpacks and returns a fixed length opaque data stream, similarly to
205   :meth:`unpack_fstring`.
206
207
208.. method:: Unpacker.unpack_string()
209
210   Unpacks and returns a variable length string.  The length of the string is first
211   unpacked as an unsigned integer, then the string data is unpacked with
212   :meth:`unpack_fstring`.
213
214
215.. method:: Unpacker.unpack_opaque()
216
217   Unpacks and returns a variable length opaque data string, similarly to
218   :meth:`unpack_string`.
219
220
221.. method:: Unpacker.unpack_bytes()
222
223   Unpacks and returns a variable length byte stream, similarly to
224   :meth:`unpack_string`.
225
226The following methods support unpacking arrays and lists:
227
228
229.. method:: Unpacker.unpack_list(unpack_item)
230
231   Unpacks and returns a list of homogeneous items.  The list is unpacked one
232   element at a time by first unpacking an unsigned integer flag.  If the flag is
233   ``1``, then the item is unpacked and appended to the list.  A flag of ``0``
234   indicates the end of the list.  *unpack_item* is the function that is called to
235   unpack the items.
236
237
238.. method:: Unpacker.unpack_farray(n, unpack_item)
239
240   Unpacks and returns (as a list) a fixed length array of homogeneous items.  *n*
241   is number of list elements to expect in the buffer. As above, *unpack_item* is
242   the function used to unpack each element.
243
244
245.. method:: Unpacker.unpack_array(unpack_item)
246
247   Unpacks and returns a variable length *list* of homogeneous items. First, the
248   length of the list is unpacked as an unsigned integer, then each element is
249   unpacked as in :meth:`unpack_farray` above.
250
251
252.. _xdr-exceptions:
253
254Exceptions
255----------
256
257Exceptions in this module are coded as class instances:
258
259
260.. exception:: Error
261
262   The base exception class.  :exc:`Error` has a single public attribute
263   :attr:`msg` containing the description of the error.
264
265
266.. exception:: ConversionError
267
268   Class derived from :exc:`Error`.  Contains no additional instance variables.
269
270Here is an example of how you would catch one of these exceptions::
271
272   import xdrlib
273   p = xdrlib.Packer()
274   try:
275       p.pack_double(8.01)
276   except xdrlib.ConversionError as instance:
277       print('packing the double failed:', instance.msg)
278
279