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diff --git a/Doc/library/xdrlib.rst b/Doc/library/xdrlib.rst new file mode 100644 index 0000000000..6339a7fcea --- /dev/null +++ b/Doc/library/xdrlib.rst @@ -0,0 +1,276 @@ + +: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 + +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 + |