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<title>3.14.7 Example
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<H2><A NAME=
"SECTION0051470000000000000000"></A><A NAME=
"pickle-example"></A>
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.
<div class=
"verbatim"><pre>
"""Print and number lines in a text file."""
def __init__(self, file):
self.lineno = self.lineno +
1
line = self.fh.readline()
return
"%d: %s" % (self.lineno, line)
odict = self.__dict__.copy() # copy the dict since we change it
del odict['fh'] # remove filehandle entry
def __setstate__(self,dict):
fh = open(dict['file']) # reopen file
count = dict['lineno'] # read from file...
while count: # until line count is restored
self.__dict__.update(dict) # update attributes
self.fh = fh # save the file object
A sample usage might be something like this:
<div class=
"verbatim"><pre>
>>> import TextReader
>>> obj = TextReader.TextReader(
"TextReader.py")
>>> obj.readline()
'
1: #!/usr/local/bin/python'
>>> # (more invocations of obj.readline() here)
>>> import pickle
>>> pickle.dump(obj,open('save.p','w'))
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.
<div class=
"verbatim"><pre>
>>> import pickle
>>> reader = pickle.load(open('save.p'))
>>> reader.readline()
'
8:
"Print and number lines in a text file."'
<p class=
"heading">See Also:
</p>
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"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 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 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 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.
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<span class=
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2.4.2, documentation updated on
28 September
2005.
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