Exclude object's field from pickling in python

Question

I would like to avoid pickling of certain fields in an instance of a class. Currently, before pickling I just set those fields to None, but I wonder whether there's more elegant solution?

Answer 1

One way to handle instance attributes that are not picklable objects is to use the special methods available for modifying a class instance's state: __getstate__() and __setstate__(). Here is an example

class Foo(object):

    def __init__(self, value, filename):
        self.value = value
        self.logfile = file(filename, 'w')

    def __getstate__(self):
        """Return state values to be pickled."""
        f = self.logfile
        return (self.value, f.name, f.tell())

    def __setstate__(self, state):
        """Restore state from the unpickled state values."""
        self.value, name, position = state
        f = file(name, 'w')
        f.seek(position)
        self.logfile = f

When an instance of Foo is pickled, Python will pickle only the values returned to it when it calls the instance's __getstate__() method. Likewise, during unpickling, Python will supply the unpickled values as an argument to the instance's __setstate__() method. Inside the __setstate__() method we are able to recreate the file object based on the name and position information we pickled, and assign the file object to the instance's logfile attribute.

Reference: http://www.ibm.com/developerworks/library/l-pypers.html