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<H2><A NAME="SECTION0051470000000000000000"></A><A NAME="pickle-example"></A>
<BR>
3.14.7 Example
</H2>

<P>
Here's a simple example of how to modify pickling behavior for a
class.  The <tt class="class">TextReader</tt> class opens a text file, and returns
the line number and line contents each time its <tt class="method">readline()</tt>
method is called. If a <tt class="class">TextReader</tt> instance is pickled, all
attributes <em>except</em> the file object member are saved. When the
instance is unpickled, the file is reopened, and reading resumes from
the last location. The <tt class="method">__setstate__()</tt> and
<tt class="method">__getstate__()</tt> methods are used to implement this behavior.

<P>
<div class="verbatim"><pre>
class TextReader:
    """Print and number lines in a text file."""
    def __init__(self, file):
        self.file = file
        self.fh = open(file)
        self.lineno = 0

    def readline(self):
        self.lineno = self.lineno + 1
        line = self.fh.readline()
        if not line:
            return None
        if line.endswith("\n"):
            line = line[:-1]
        return "%d: %s" % (self.lineno, line)

    def __getstate__(self):
        odict = self.__dict__.copy() # copy the dict since we change it
        del odict['fh']              # remove filehandle entry
        return odict

    def __setstate__(self,dict):
        fh = open(dict['file'])      # reopen file
        count = dict['lineno']       # read from file...
        while count:                 # until line count is restored
            fh.readline()
            count = count - 1
        self.__dict__.update(dict)   # update attributes
        self.fh = fh                 # save the file object
</pre></div>

<P>
A sample usage might be something like this:

<P>
<div class="verbatim"><pre>
&gt;&gt;&gt; import TextReader
&gt;&gt;&gt; obj = TextReader.TextReader("TextReader.py")
&gt;&gt;&gt; obj.readline()
'1: #!/usr/local/bin/python'
&gt;&gt;&gt; # (more invocations of obj.readline() here)
... obj.readline()
'7: class TextReader:'
&gt;&gt;&gt; import pickle
&gt;&gt;&gt; pickle.dump(obj,open('save.p','w'))
</pre></div>

<P>
If you want to see that <tt class="module"><a href="module-pickle.html">pickle</a></tt> works across Python
processes, start another Python session, before continuing.  What
follows can happen from either the same process or a new process.

<P>
<div class="verbatim"><pre>
&gt;&gt;&gt; import pickle
&gt;&gt;&gt; reader = pickle.load(open('save.p'))
&gt;&gt;&gt; reader.readline()
'8:     "Print and number lines in a text file."'
</pre></div>

<P>
<div class="seealso">
  <p class="heading">See Also:</p>

  <dl compact="compact" class="seemodule">
    <dt>Module <b><tt class="module"><a href="module-copyreg.html">copy_reg</a></tt>:</b>
    <dd>Pickle interface constructor
                                registration for extension types.
  </dl>

<P>
<dl compact="compact" class="seemodule">
    <dt>Module <b><tt class="module"><a href="module-shelve.html">shelve</a></tt>:</b>
    <dd>Indexed databases of objects; uses <tt class="module">pickle</tt>.
  </dl>

<P>
<dl compact="compact" class="seemodule">
    <dt>Module <b><tt class="module"><a href="module-copy.html">copy</a></tt>:</b>
    <dd>Shallow and deep object copying.
  </dl>

<P>
<dl compact="compact" class="seemodule">
    <dt>Module <b><tt class="module"><a href="module-marshal.html">marshal</a></tt>:</b>
    <dd>High-performance serialization of built-in types.
  </dl>
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<span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span>
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