shallow vs deep copies in python

shallowDeepCopy.py

# copy lists in python
a = [1, 2, [3],[[4]]]
print('a'.ljust(20, ' '),id(a), id(a[2]), id(a[2][0]), id(a[3][0]))
# not a copy
b = a
print('b (not a copy)'.ljust(20, ' '), id(b), id(b[2]), id(b[2][0]), id(b[3][0]))
# a shallow copy I
b = a[:]
print('b (shallow copy I)'.ljust(20, ' '), id(b), id(b[2]), id(b[2][0]), id(b[3][0]))
# a shallow copy II
b = list(a)
print('b (shallow copy II)'.ljust(20, ' '), id(b), id(b[2]), id(b[2][0]), id(b[3][0]))
# a shallow copy III
b = a.copy()
print('b (shallow copy III)'.ljust(20, ' '), id(b), id(b[2]), id(b[2][0]), id(b[3][0]))
# a shallow copy IV
from copy import copy
b = copy(a)
print('b (shallow copy IV)'.ljust(20, ' '), id(b), id(b[2]), id(b[2][0]), id(b[3][0]))
# a deep copy
from copy import deepcopy
b = deepcopy(a)
print('b (deep copy)'.ljust(20, ' '), id(b), id(b[2]), id(b[2][0]), id(b[3][0]))

# This returns
# a                    2609788978112 2609788977728 140725089937248 2609788979520
# b (not a copy)       2609788978112 2609788977728 140725089937248 2609788979520
# b (shallow copy I)   2610139993984 2609788977728 140725089937248 2609788979520
# b (shallow copy II)  2609788977216 2609788977728 140725089937248 2609788979520
# b (shallow copy III) 2610139999616 2609788977728 140725089937248 2609788979520
# b (shallow copy IV)  2610139827328 2609788977728 140725089937248 2609788979520
# b (deep copy)        2610139993984 2609788977216 140725089937248 2609788846272


# Slicing in python always creates shallow copies. This is proven by the code
data = [1, 2, [3,33,333] ,4, 5]
s = slice(0, 3, 2)
data2 = data[s]
print('data2 (prior to modification)', data2)
data2[0] = -2000
data2[1][0] = -3000
print('data2 (after modification)', data2)
print('data (after modification)', data)

# that returns
# data2 (prior to modification) [1, [3, 33, 333]]
# data2 (after modification) [-2000, [-3000, 33, 333]]
# data (after modification) [1, 2, [-3000, 33, 333], 4, 5]

With deep copy the address of an immutable object remains the same. However, the address of the mutable container of the immutable object changes which means that a new assignment will not change the original.

This is a good read https://nedbatchelder.com/text/names.html