Deep Copy vs. Shallow: Definitions

copy.copy() (shallow copy)
A shallow copy creates a new outer object and copies the references of the nested objects.

import copy

lst = [[1, 2], [3, 4]]  
shallow = copy.copy(lst)

shallow[0][0] = 999
print(lst)   # [[999, 2], [3, 4]]

copy.deepcopy() (deep copy)
A deep copy creates a new outer object AND new copies of all nested objects, creating new references (new memory addresses) for each.

import copy

lst = [[1, 2], [3, 4]]
deep = copy.deepcopy(lst)

deep[0][0] = 999
print(lst)      # [[1, 2], [3, 4]]