| :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 |
| |
| **Source code:** :source:`Lib/xdrlib.py` |
| |
| -------------- |
| |
| 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 as instance: |
| print('packing the double failed:', instance.msg) |
| |