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<title>3.14.7 Example </title>
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<H2><A NAME="SECTION0051470000000000000000"><!--x--></A><A NAME="pickle-example"><!--z--></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 <i>except</i> 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>
>>> import TextReader
>>> obj = TextReader.TextReader("TextReader.py")
>>> obj.readline()
'1: #!/usr/bin/python'
>>> # (more invocations of obj.readline() here)
... obj.readline()
'7: class TextReader:'
>>> import pickle
>>> 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>
>>> import pickle
>>> reader = pickle.load(open('save.p'))
>>> reader.readline()
'8: "Print and number lines in a text file."'
</pre></div>
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<p class="heading"><b>See Also:</b></p>
<dl 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.
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<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 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 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.
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<span class="release-info">Release 2.3.4, documentation updated on May 20, 2004.</span>
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