Python in a Nutshell

Name

deepcopy

Synopsis

deepcopy(x,[memo])

Makes a deep copy of x and returns it. Deep copying implies a recursive walk over a directed graph of references. A precaution is needed to preserve the graph’s shape: when references to the same object are met more than once during the walk, distinct copies must not be made. Rather, references to the same copied object must be used. Consider the following simple example:

sublist = [1,2]
original = [sublist, sublist]
thecopy = copy.deepcopy(original)

original[0] is original[1] is True (i.e., the two items of list original refer to the same object). This is an important property of original and therefore must be preserved in anything that claims to be a copy of it. The semantics of copy.deepcopy are defined to ensure that thecopy[0] is thecopy[1] is also True in this case. In other words, the shapes of the graphs of references of original and thecopy are the same. Avoiding repeated copying has an important beneficial side effect: preventing infinite loops that would otherwise occur if the graph has cycles.

copy.deepcopy accepts a second, optional argument memo, which is a dictionary that maps the id( ) of objects already copied to the new objects that are their copies. memo is passed by recursive calls of deepcopy to itself, but you may also explicitly pass it (normally as an originally empty dictionary) if you need to keep such a correspondence map between the identities of originals and copies of objects.

A class can customize the way copy.deepcopy ...