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(python2.1-lib.info)Example 3

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Enter node , (file) or (file)node

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

     # illustrate __setstate__ and __getstate__  methods
     # used in pickling.
     
     class TextReader:
         "Print and number lines in a text file."
         def __init__(self,file):
             self.file = file
             self.fh = open(file,'r')
             self.lineno = 0
     
         def readline(self):
             self.lineno = self.lineno + 1
             line = self.fh.readline()
             if not line:
                 return None
             return "%d: %s" % (self.lineno,line[:-1])
     
         # return data representation for pickled object
         def __getstate__(self):
             odict = self.__dict__    # get attribute dictionary
             del odict['fh']          # remove filehandle entry
             return odict
     
         # restore object state from data representation generated
         # by __getstate__
         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
             dict['fh'] = fh          # create filehandle entry
             self.__dict__ = dict     # make dict our attribute dictionary

A sample usage might be something like this:

     >>> import TextReader
     >>> obj = TextReader.TextReader("TextReader.py")
     >>> obj.readline()
     '1: #!/usr/local/bin/python'
     >>> # (more invocations of obj.readline() here)
     ... obj.readline()
     '7: class TextReader:'
     >>> import pickle
     >>> pickle.dump(obj,open('save.p','w'))
     
       (start another Python session)
     
     >>> import pickle
     >>> reader = pickle.load(open('save.p'))
     >>> reader.readline()
     '8:     "Print and number lines in a text file."'