13.4. xdrlib — Encode and decode XDR data
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.
- Packer is the class for packing data into XDR representation. The
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.
- 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.
13.4.1. Packer Objects
Packer instances have the following methods:
- Returns the current pack buffer as a string.
- 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().
- Packs the single-precision floating point number value.
- Packs the double-precision floating point number value.
The following methods support packing strings, bytes, and opaque data:
- 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.
- Packs a fixed length opaque data stream, similarly to pack_fstring().
- 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
- Packs a variable length opaque data string, similarly to pack_string().
- Packs a variable length byte stream, similarly to pack_string().
The following methods support packing arrays and lists:
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:
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.
- 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
13.4.2. Unpacker Objects
The Unpacker class offers the following methods:
- Resets the string buffer with the given data.
- Returns the current unpack position in the data buffer.
- Sets the data buffer unpack position to position. You should be careful about
using get_position() and set_position().
- Returns the current unpack data buffer as a string.
- 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.
- Unpacks a single-precision floating point number.
- Unpacks a double-precision floating point number, similarly to
In addition, the following methods unpack strings, bytes, and opaque data:
- 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.
- Unpacks and returns a fixed length opaque data stream, similarly to
- 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
- Unpacks and returns a variable length opaque data string, similarly to
- Unpacks and returns a variable length byte stream, similarly to
The following methods support unpacking arrays and lists:
- 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.
- 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.
- 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.
Exceptions in this module are coded as class instances:
- The base exception class. Error has a single public data member
msg containing the description of the error.
- Class derived from Error. Contains no additional instance variables.
Here is an example of how you would catch one of these exceptions:
p = xdrlib.Packer()
except xdrlib.ConversionError as instance:
print('packing the double failed:', instance.msg)