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