Can Pickle handle multiple object references

Question

If I have objects a and b and both reference object obj, what happens when I Pickle and then restore the objects? Will the pickled data 'know' that a and b both referenced the same object and restore everything accordingly, or will the two get two different — and initially equal — objects?

Answer 1

Yes, shared objects will only get serialized once (the pickle protocol can even handle circular references).

From the documentation:

The pickle module keeps track of the objects it has already serialized, so that later references to the same object won’t be serialized again. 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. 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.

Answer 2

As @aix points out, pickle understands multiple references to the same object, but only within a single pickling. That is, pickle always pickles a single object. If that object has references within it, those references will be properly shared in the unpickled object.

But if you call pickle twice, to pickle two objects, shared references between the objects will not be preserved properly. The object will now exist twice.