* The :mod:`pickle` module keeps track of the objects it has already serialized,

so that later references to the same object won't be serialized again.

:mod:`marshal` doesn't do this.

This has implications both for recursive objects and object sharing. Recursive

objects are objects that contain references to themselves. These are not

handled by marshal, and in fact, attempting to marshal recursive objects will

crash your Python interpreter. Object sharing happens when there are multiple

references to the same object in different places in the object hierarchy being

serialized. :mod:`pickle` stores such objects only once, and ensures that all

other references point to the master copy. Shared objects remain shared, which

can be very important for mutable objects.