Python in a Nutshell, 2nd Edition

The copy Module

As discussed in Assignment Statements, assignment in Python does not copy the righthand side object being assigned. Rather, assignment adds a reference to the righthand side object. When you want a copy of object x, you can ask x for a copy of itself, or you can ask x’s type to make a new instance copied from x. If x is a list, list(x) returns a copy of x, as does x[:]. If x is a dictionary, dict(x) and x.copy( ) return a copy of x. If x is a set, set(x) and x.copy( ) return a copy of x. In each of these cases, my strong personal preference is for the uniform and readable idiom of calling the type, but there is no consensus on this style issue in the community of Python experts.

The copy module supplies a copy function to create and returns a copy of many types of objects. Normal copies, such as list(x) for a list x and copy.copy(x), are also known as shallow copies: when x has references to other objects (as items or attributes), a normal (shallow) copy of x has distinct references to the same objects. Sometimes, however, you need a deep copy, where referenced objects are copied recursively; fortunately, this requirement is rare, because a deep copy can take a lot of memory and time. Module copy supplies a deepcopy(x) function to create and return a deep copy.

copy	copy(x) Creates and returns a shallow copy of x, for x of many types (copies of several types, such as modules, classes, files, frames, and other internal types, are, however, not supported). If x is immutable, ...