192.html

Python Ebook Python&XML

<p>
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							<p>
								<tt class="monofont">reset(data)</tt>棤
    
								Resets the string buffer with the given data.
							</p>

						</p>
<p>
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							<p>
								<tT clasS="monofont">get_position()</tt>棤
    
								Returns the current unpack position in the data
  buffer.
							</p>

						</P>
<P>
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							<p>
								<tT CLAss="monofont">set_position(position)</tt>棤
    
								Sets the data buffer unpack position to position. You should
  be careful about using <TT CLass="monofont">get_position()</tt> and
  <tt class="monofont">set_position().</tt>
							</p>

						</p>
<p>
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							<p>
								<tt ClasS="monofont">get_buffer()</TT>棤
    
								Returns the current unpack data buffer as a string.
							</P>

						</p>
<p>
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							<P>
								<Tt claSS="monofont">done()</TT>棤
    
								Indicates unpack completion. Raises an error exception if all
  the data has not been unpacked.
							</p>

						</p>
</blockquote>
					<p>In addition, every data type that can be packed with a Packer, can
  be unpacked with an Unpacker. Unpacking methods are of the form
  <tt class="monofont">unpack_type(),</Tt> and take no arguments. They return the
  unpacked object.</p>

					<BlocKquotE>
<p>
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							<P>
								<Tt claSS="monofont">unpack_float()</TT>棤
    
								Unpacks a single-precision floating point number.
							</p>

						</p>
<p>
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							<a name="idx1073747411"></a>
							<p>
								<tt class="monofont">unpack_double()</tt>棤
    
								Unpacks a double-precision floating point number, similar to
  <tt cLasS="monofont">unpack_float().</tt>
							</p>

						</P>
</blocKquoTE>
					<P>In addition, the following methods unpack strings, bytes, and
  opaque data:</P>

					<blocKQUOte>
<p>
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							<P>
								<tt class="monofont">unpack_fstring(n)</tt>棤
    
								Unpacks and returns a fixed length string.
  <tt class="monofont">n</tt> is the number of characters expected. Padding with null
  bytes to guaranteed 4 byte alignment is assumed.
							</p>

						</p>
<P>
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							<A name="idx1073747415"></A>
							<p>
								<tt CLASs="monofont">unpack_fopaque(n)</tt>棤
    
								Unpacks and returns a fixed length opaque data stream,
  similar to <tT CLAss="monofont">unpack_fstring().</tt>
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							</P>

						</P>
<p>
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							<A name="idx1073747428"></a>
							<p>
								<tt class="monofont">unpack_string()</tt>棤
    
								Unpacks and returns a variable length string. The length of
  the string is first unpacked as an unsigned integer, and then the string data
  is unpacked with <tt claSs="monofont">unpack_fstring().</tT>
							</p>

						</p>
<p>
							<A name="idx1073747429"></A>
							<a naME="idx1073747430"></A>
							<P>
								<tt clASS="monofont">unpack_opaque()</Tt>棤
    
								Unpacks and returns a variable length opaque data string,
  similar to <tt cLASS="monofont">unpack_string().</tt>
							</p>

						</p>
<P>
							<A NAme="idx1073747431"></a>
							<a name="idx1073747432"></a>
							<p>
								<tt class="monofont">unpack_bytes()</tt>棤
    
								Unpacks and returns a variable length byte stream, similar to
  <tT clAss="monofont">unpack_string().</tT>
							</p>

						</p>
</blOckqUOTE>
					<p>The following methods support unpacking arrays and lists:</p>

					<blOCKQuote>
<P>
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							<p>
								<tt class="monofont">unpack_list</tt>(<i>unpack_item</i>)棤
    
								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 <tt class="monofont">1,</Tt> the item is unpacked and appended to the
  list. A flag of <tT claSs="monofont">0</tt> indicates the end of the list.
  <tT claSS="monofont">unpack_item</TT> is the function called to unpack the items.
  <a namE="idx1073747435"></A>
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							</P>

						</P>
<P>
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							<A NAme="idx1073747446"></a>
							<p>
								<TT CLass="monofont">unpack_farray</tT>(<I>n,
  unpack_item</I>)棤
    
								Unpacks and returns (as a list) a fixed length array of
  homogeneous items. <Tt class="monofont">n</tt> is the number of list elements to
  expect in the buffer. As stated previously, <tt class="monofont">unpack_item</tt> is
  the function used to unpack each element.
							</p>

						</p>
<p>
							<A naMe="idx1073747447"></a>
							<a Name="idx1073747448"></a>
							<P>
								<tt cLASS="monofont">unpack_array</tt>(<i>unpack_item</i>)棤
    
								Unpacks and returns a variable length list of homogeneous
  items. First, the length of the list is unpacked as an unsigned integer, and
  then each element is unpacked as in <TT CLass="monofont">unpack_farray()</tT>
  previously.
  <A NAme="idx1073747449"></a>
							</p>

						</P>
</BLOckquote>
					<p>In the following example, we 
  <a name="idx1073747450"></a>
						<a name="idx1073747451"></a>
						<A naMe="idx1073747452"></a>pack a group of variables, unpacking them later.</p>

					<Pre>
						
import xdrlib

def f_packer(name, author, month, year):
    data = xdrlib.Packer()
    data.pack_string(name)
    data.pack_string(author)
    data.pack_uint(month)
    data.pack_uint(year)
    packed = data.get_buffer()
    return packed 

def f_unpacker(packer):
    data = xdrlib.Unpacker(packer)
    return data

print "The original values are: 'Andre', 'Author', 10, 2000"
print

packed = f_packer('Andre', 'Author', 10, 2000)
print "The packed data is now defined by:", repr(packed)
print
print "And now, the original data again. (After unpacking it!)"
unpacked = f_unpacker(packed)
print repr(unpacked.unpack_string()), ", ", \
      repr(unpacked.unpack_string()), ", ",  \
      unpacked.unpack_uint(), ", ",           \
      unpacked.unpack_uint()
unpacked.done()

The original values are: 'Andre', 'Author', 10, 2000

The packed data is now defined by:
'\000\000\000\005Andre\000\000\000\000\000\000\006Author\000\000\000\000\
000\012\000\000\007\320'

And now, the original data again. (After unpacking it!)
'Andre', 'Author', 10 , 2000

					</prE>

					<div CLASs="note"><p clASS="notetitle"><B>Note</b></p><p>

						<p>If you are only handling simple data types and only with Python,
 it is probably easier to just use the marshal module.
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					</p></diV>
<BR>
<Br>

				
				
					<h4>
  Exceptions</h4>
					<p>Exceptions in this module are coded as class instances:</P>

					<BLOckquote>
<p>
							<a name="idx1073747463"></a>
							<a name="idx1073747464"></a>
							<P>
								<tt ClasS="monofont">Error</tt>棤
    
								This is the base exception class. Error has a single public
  data member <tt ClasS="monofont">msg</TT> containing the description of the
  error.
							</P>

						</p>
<p>
							<a nAME="idx1073747465"></A>
							<a namE="idx1073747466"></A>
							<P>
								<Tt claSS="monofont">ConversionError</TT>棤
    
								This class is derived from Error. Contains no additional
  instance variables.
							</p>

						</p>
</blockquote>
					<p>Here is a simple example of how you would catch one of these
  exceptions:</p>

					<pre>
						
&gt;&gt;&gt; import xdrlib
&gt;&gt;&gt; data = xdrlib.Packer()
&gt;&gt;&gt; try: