Question

After reading pickle documentation, I got an impression that a class needs to implement either __reduce__ or __getstate__ to get pickled correctly. But how pickling of dictionaries work then? They don’t have any of those attributes:

> dict(a=1).__reduce__()

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/home/daniyar/work/Apr24/<ipython-input-30-bc1cbd43305b> in <module>()
----> 1 dict(a=1).__reduce__()

/usr/lib/python2.6/copy_reg.pyc in _reduce_ex(self, proto)
     68     else:
     69         if base is self.__class__:
---> 70             raise TypeError, "can't pickle %s objects" % base.__name__
     71         state = base(self)
     72     args = (self.__class__, base, state)

TypeError: can't pickle dict objects



> dict(a=1).__getstate__()

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
/home/daniyar/work/Apr24/<ipython-input-31-00932fb40067> in <module>()
----> 1 dict(a=1).__getstate__()

AttributeError: 'dict' object has no attribute '__getstate__'

Also, how classes derived from dict are pickled?

Answer 1

The pickle module handles a number of types “natively”. The types it doesn’t handle natively will need to implement the “pickle protocol”. Dicts, and simple subclasses, are handled natively.