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278 lines
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ReStructuredText
278 lines
7.9 KiB
ReStructuredText
:mod:`xdrlib` --- Encode and decode XDR data
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============================================
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.. module:: xdrlib
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:synopsis: Encoders and decoders for the External Data Representation (XDR).
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**Source code:** :source:`Lib/xdrlib.py`
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.. index::
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single: XDR
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single: External Data Representation
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--------------
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The :mod:`xdrlib` module supports the External Data Representation Standard as
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described in :rfc:`1014`, written by Sun Microsystems, Inc. June 1987. It
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supports most of the data types described in the RFC.
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The :mod:`xdrlib` module defines two classes, one for packing variables into XDR
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representation, and another for unpacking from XDR representation. There are
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also two exception classes.
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.. class:: Packer()
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:class:`Packer` is the class for packing data into XDR representation. The
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:class:`Packer` class is instantiated with no arguments.
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.. class:: Unpacker(data)
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``Unpacker`` is the complementary class which unpacks XDR data values from a
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string buffer. The input buffer is given as *data*.
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.. seealso::
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:rfc:`1014` - XDR: External Data Representation Standard
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This RFC defined the encoding of data which was XDR at the time this module was
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originally written. It has apparently been obsoleted by :rfc:`1832`.
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:rfc:`1832` - XDR: External Data Representation Standard
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Newer RFC that provides a revised definition of XDR.
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.. _xdr-packer-objects:
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Packer Objects
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--------------
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:class:`Packer` instances have the following methods:
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.. method:: Packer.get_buffer()
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Returns the current pack buffer as a string.
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.. method:: Packer.reset()
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Resets the pack buffer to the empty string.
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In general, you can pack any of the most common XDR data types by calling the
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appropriate ``pack_type()`` method. Each method takes a single argument, the
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value to pack. The following simple data type packing methods are supported:
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:meth:`pack_uint`, :meth:`pack_int`, :meth:`pack_enum`, :meth:`pack_bool`,
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:meth:`pack_uhyper`, and :meth:`pack_hyper`.
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.. method:: Packer.pack_float(value)
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Packs the single-precision floating point number *value*.
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.. method:: Packer.pack_double(value)
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Packs the double-precision floating point number *value*.
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The following methods support packing strings, bytes, and opaque data:
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.. method:: Packer.pack_fstring(n, s)
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Packs a fixed length string, *s*. *n* is the length of the string but it is
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*not* packed into the data buffer. The string is padded with null bytes if
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necessary to guaranteed 4 byte alignment.
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.. method:: Packer.pack_fopaque(n, data)
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Packs a fixed length opaque data stream, similarly to :meth:`pack_fstring`.
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.. method:: Packer.pack_string(s)
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Packs a variable length string, *s*. The length of the string is first packed
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as an unsigned integer, then the string data is packed with
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:meth:`pack_fstring`.
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.. method:: Packer.pack_opaque(data)
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Packs a variable length opaque data string, similarly to :meth:`pack_string`.
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.. method:: Packer.pack_bytes(bytes)
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Packs a variable length byte stream, similarly to :meth:`pack_string`.
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The following methods support packing arrays and lists:
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.. method:: Packer.pack_list(list, pack_item)
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Packs a *list* of homogeneous items. This method is useful for lists with an
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indeterminate size; i.e. the size is not available until the entire list has
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been walked. For each item in the list, an unsigned integer ``1`` is packed
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first, followed by the data value from the list. *pack_item* is the function
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that is called to pack the individual item. At the end of the list, an unsigned
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integer ``0`` is packed.
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For example, to pack a list of integers, the code might appear like this::
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import xdrlib
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p = xdrlib.Packer()
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p.pack_list([1, 2, 3], p.pack_int)
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.. method:: Packer.pack_farray(n, array, pack_item)
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Packs a fixed length list (*array*) of homogeneous items. *n* is the length of
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the list; it is *not* packed into the buffer, but a :exc:`ValueError` exception
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is raised if ``len(array)`` is not equal to *n*. As above, *pack_item* is the
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function used to pack each element.
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.. method:: Packer.pack_array(list, pack_item)
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Packs a variable length *list* of homogeneous items. First, the length of the
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list is packed as an unsigned integer, then each element is packed as in
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:meth:`pack_farray` above.
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.. _xdr-unpacker-objects:
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Unpacker Objects
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----------------
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The :class:`Unpacker` class offers the following methods:
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.. method:: Unpacker.reset(data)
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Resets the string buffer with the given *data*.
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.. method:: Unpacker.get_position()
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Returns the current unpack position in the data buffer.
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.. method:: Unpacker.set_position(position)
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Sets the data buffer unpack position to *position*. You should be careful about
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using :meth:`get_position` and :meth:`set_position`.
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.. method:: Unpacker.get_buffer()
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Returns the current unpack data buffer as a string.
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.. method:: Unpacker.done()
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Indicates unpack completion. Raises an :exc:`Error` exception if all of the
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data has not been unpacked.
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In addition, every data type that can be packed with a :class:`Packer`, can be
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unpacked with an :class:`Unpacker`. Unpacking methods are of the form
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``unpack_type()``, and take no arguments. They return the unpacked object.
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.. method:: Unpacker.unpack_float()
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Unpacks a single-precision floating point number.
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.. method:: Unpacker.unpack_double()
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Unpacks a double-precision floating point number, similarly to
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:meth:`unpack_float`.
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In addition, the following methods unpack strings, bytes, and opaque data:
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.. method:: Unpacker.unpack_fstring(n)
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Unpacks and returns a fixed length string. *n* is the number of characters
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expected. Padding with null bytes to guaranteed 4 byte alignment is assumed.
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.. method:: Unpacker.unpack_fopaque(n)
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Unpacks and returns a fixed length opaque data stream, similarly to
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:meth:`unpack_fstring`.
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.. method:: Unpacker.unpack_string()
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Unpacks and returns a variable length string. The length of the string is first
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unpacked as an unsigned integer, then the string data is unpacked with
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:meth:`unpack_fstring`.
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.. method:: Unpacker.unpack_opaque()
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Unpacks and returns a variable length opaque data string, similarly to
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:meth:`unpack_string`.
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.. method:: Unpacker.unpack_bytes()
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Unpacks and returns a variable length byte stream, similarly to
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:meth:`unpack_string`.
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The following methods support unpacking arrays and lists:
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.. method:: Unpacker.unpack_list(unpack_item)
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Unpacks and returns a list of homogeneous items. The list is unpacked one
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element at a time by first unpacking an unsigned integer flag. If the flag is
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``1``, then the item is unpacked and appended to the list. A flag of ``0``
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indicates the end of the list. *unpack_item* is the function that is called to
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unpack the items.
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.. method:: Unpacker.unpack_farray(n, unpack_item)
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Unpacks and returns (as a list) a fixed length array of homogeneous items. *n*
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is number of list elements to expect in the buffer. As above, *unpack_item* is
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the function used to unpack each element.
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.. method:: Unpacker.unpack_array(unpack_item)
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Unpacks and returns a variable length *list* of homogeneous items. First, the
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length of the list is unpacked as an unsigned integer, then each element is
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unpacked as in :meth:`unpack_farray` above.
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.. _xdr-exceptions:
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Exceptions
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----------
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Exceptions in this module are coded as class instances:
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.. exception:: Error
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The base exception class. :exc:`Error` has a single public attribute
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:attr:`msg` containing the description of the error.
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.. exception:: ConversionError
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Class derived from :exc:`Error`. Contains no additional instance variables.
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Here is an example of how you would catch one of these exceptions::
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import xdrlib
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p = xdrlib.Packer()
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try:
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p.pack_double(8.01)
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except xdrlib.ConversionError as instance:
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print('packing the double failed:', instance.msg)
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