##
3.8.1 Example

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."'