Code source : Lib/xdrlib.py
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¶
- class xdrlib.Unpacker(data)¶
Unpackeris 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.
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
pack_type() method. Each method takes a single argument, the
value to pack. The following simple data type packing methods are supported:
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:
- 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.
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
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
1is 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
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
ValueErrorexception is raised if
len(array)is not equal to n. As above, pack_item is the function used to pack each element.
Unpacker class offers the following methods:
Resets the string buffer with the given data.
Returns the current unpack position in the data buffer.
Returns the current unpack data buffer as a string.
Indicates unpack completion. Raises an
Errorexception 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
0indicates 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.
Exceptions in this module are coded as class instances:
- exception xdrlib.Error¶
The base exception class.
Errorhas a single public attribute
msgcontaining 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)