13.5. "xdrlib" — Encode and decode XDR data
*******************************************

**Source code:** Lib/xdrlib.py

======================================================================

The "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 "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 xdrlib.Packer

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

class xdrlib.Unpacker(data)

   "Unpacker" is the complementary class which unpacks XDR data values
   from a string buffer.  The input buffer is given as *data*.

See also:

  **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.


13.5.1. Packer Objects
======================

"Packer" instances have the following methods:

Packer.get_buffer()

   Returns the current pack buffer as a string.

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: "pack_uint()", "pack_int()",
"pack_enum()", "pack_bool()", "pack_uhyper()", and "pack_hyper()".

Packer.pack_float(value)

   Packs the single-precision floating point number *value*.

Packer.pack_double(value)

   Packs the double-precision floating point number *value*.

The following methods support packing strings, bytes, and opaque data:

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.

Packer.pack_fopaque(n, data)

   Packs a fixed length opaque data stream, similarly to
   "pack_fstring()".

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 "pack_fstring()".

Packer.pack_opaque(data)

   Packs a variable length opaque data string, similarly to
   "pack_string()".

Packer.pack_bytes(bytes)

   Packs a variable length byte stream, similarly to "pack_string()".

The following methods support packing arrays and lists:

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)

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
   "ValueError" exception is raised if "len(array)" is not equal to
   *n*.  As above, *pack_item* is the function used to pack each
   element.

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 "pack_farray()" above.


13.5.2. Unpacker Objects
========================

The "Unpacker" class offers the following methods:

Unpacker.reset(data)

   Resets the string buffer with the given *data*.

Unpacker.get_position()

   Returns the current unpack position in the data buffer.

Unpacker.set_position(position)

   Sets the data buffer unpack position to *position*.  You should be
   careful about using "get_position()" and "set_position()".

Unpacker.get_buffer()

   Returns the current unpack data buffer as a string.

Unpacker.done()

   Indicates unpack completion.  Raises an "Error" exception if all of
   the data has not been unpacked.

In addition, every data type that can be packed with a "Packer", can
be unpacked with an "Unpacker".  Unpacking methods are of the form
"unpack_type()", and take no arguments.  They return the unpacked
object.

Unpacker.unpack_float()

   Unpacks a single-precision floating point number.

Unpacker.unpack_double()

   Unpacks a double-precision floating point number, similarly to
   "unpack_float()".

In addition, the following methods unpack strings, bytes, and opaque
data:

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.

Unpacker.unpack_fopaque(n)

   Unpacks and returns a fixed length opaque data stream, similarly to
   "unpack_fstring()".

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 "unpack_fstring()".

Unpacker.unpack_opaque()

   Unpacks and returns a variable length opaque data string, similarly
   to "unpack_string()".

Unpacker.unpack_bytes()

   Unpacks and returns a variable length byte stream, similarly to
   "unpack_string()".

The following methods support unpacking arrays and lists:

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.

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.

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 "unpack_farray()" above.


13.5.3. Exceptions
==================

Exceptions in this module are coded as class instances:

exception xdrlib.Error

   The base exception class.  "Error" has a single public attribute
   "msg" containing the description of the error.

exception xdrlib.ConversionError

   Class derived from "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
