What do I put in the visit dict passed to my user-defined __deepcopy__ function?

I need to define开发者_开发知识库 my own deepcopy function for one of my classes. The documentation says that the function __deepcopy__() is passed a memo dictionary for tracking which objects have already been copied, to avoid getting caught in coping a recursive object. However it doesn't say what to put in the dictionary. Do I put objects in, or object IDs? How do you use it? I can't find any site that explains it, except this book that says that it need not be used, except to pass it to other invocations of __deepcopy__.

Thanks

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To be consistent with the built-in copy.deepcopy behavior, you should use the id of the object as a key.

More Details:

While you can often get away with using a different key scheme (such as the object itself, if it's hashable), as long as you're consistent, you can still run into trouble. Frequently __deepcopy__ is implemented by calling copy.deepcopy on some contained attributes. If you add items directly to memo, you have to be certain the keys won't collide with items added by copy.deepcopy, which adds keys generated by id, which are integers.

So if you use a different key scheme, such as using objects directly as keys, you can't ever add any integer objects.

Bottom Line:

It's much easier to just use id and not have to worry about the exceptions above.

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To answer my own question, with thanks to @Jon-Eric:

What I understand from the example shown at http://www.doughellmann.com/PyMOTW/copy/, is that you use self as the key, and the copied object as the value. Then, when it is attempted to copy this object(self) again, one can check the memo dictionary to see whether it has already been copied, and return the reference to the copy, memo[self].