The difference between shallow and deep copying is only relevant for compound objects (objects that contain other objects, like lists or class instances):
Two problems often exist with deep copy operations that don't exist with shallow copy operations:
The deepcopy() function avoids these problems by:
This version does not copy types like module, class, function, method, stack trace, stack frame, file, socket, window, array, or any similar types.
Classes can use the same interfaces to control copying that they use to control pickling: they can define methods called __getinitargs__(), __getstate__() and __setstate__(). See the description of module pickle for information on these methods. The copy module does not use the copy_reg registration module.
In order for a class to define its own copy implementation, it can define special methods __copy__() and __deepcopy__(). The former is called to implement the shallow copy operation; no additional arguments are passed. The latter is called to implement the deep copy operation; it is passed one argument, the memo dictionary. If the __deepcopy__() implementation needs to make a deep copy of a component, it should call the deepcopy() function with the component as first argument and the memo dictionary as second argument.