Python List Copying: References, Shallow Copies, and Deep Copies

5 January 2025

For those curious about Lists in Python, here are some interesting ideas I've come across while exploring alternative ways to copy lists.

Assignment Operator

When a list is copied using the assignment operator a = b, the copied list only contains a reference to the memory address of the original list object. Therefore, changing the original list changes the copied list as well.

Shallow Copy using List Slicing

A shallow copy can be created through list slicing:

copy_list = orig_list[:]

This creates a real copy of all top-level elements and saves them in a new memory address. Changes to top-level elements in the original list won't affect the copy. However, nested lists are still referenced, not copied. Hence "shallow" copy.

Deep Copy using copy.deepcopy()

For a true independent copy, including nested structures:

copy_list = copy.deepcopy(orig_list)

This iterates through all elements, including nested lists, creating real copies at new memory addresses. Hence "deep" copy.