Shallow Copy & Deep Copy in Python
Buy Me a Coffee☕
*My post explains variable assignment in Python.
### Normal Copy:
*Memos:
* The reference of a list is stored in a variable.
* `v1` and `v2` have the same references of the same shallow list.
* `v1` and `v2` have the same references of the same deep list.
#### Shallow list ####
# ↓↓↓↓↓↓↓↓↓↓↓ ↓
v1 = ['a', 'b', ['c', 'd']] # Equivalent
v2 = v1 # ↑↑↑↑↑↑↑↑↑↑ # v1 = v2 = ['a', 'b', ['c', 'd']]
# Deep list
print(v1, v2) # ['a', 'b', ['c', 'd']] ['a', 'b', ['c', 'd']]
v2[1] = 'B'
v2[2][0] = 'C'
# ↓↓↓ ↓↓↓ ↓↓↓ ↓↓↓
print(v1, v2) # ['a', 'B', ['C', 'd']] ['a', 'B', ['C', 'd']]
### Shallow Copy:
*Memos:
* copy() can be used for shallow copy.
* `v1` and `v2` have the different references of the different shallow lists.
* `v1` and `v2` have the same references of the same deep list.
v1 = ['a', 'b', ['c', 'd']]
v2 = v1.copy()
print(v1, v2) # ['a', 'b', ['c', 'd']] ['a', 'b', ['c', 'd']]
v2[1] = 'B'
v2[2][0] = 'C'
# ↓↓↓ ↓↓↓ ↓↓↓
print(v1, v2) # ['a', 'b', ['C', 'd']] ['a', 'B', ['C', 'd']]
*The below with copy() is equivalent to the above.
from copy import copy
v1 = ['a', 'b', ['c', 'd']]
v2 = copy(v1)
print(v1, v2) # ['a', 'b', ['c', 'd']] ['a', 'b', ['c', 'd']]
v2[1] = 'B'
v2[2][0] = 'C'
# ↓↓↓ ↓↓↓ ↓↓↓
print(v1, v2) # ['a', 'b', ['C', 'd']] ['a', 'B', ['C', 'd']]
*The below with list() is equivalent to the above.
v1 = ['a', 'b', ['c', 'd']]
v2 = list(v1)
print(v1, v2) # ['a', 'b', ['c', 'd']] ['a', 'b', ['c', 'd']]
v2[1] = 'B'
v2[2][0] = 'C'
# ↓↓↓ ↓↓↓ ↓↓↓
print(v1, v2) # ['a', 'b', ['C', 'd']] ['a', 'B', ['C', 'd']]
### Deep Copy:
*Memos:
* deepcopy() can be used for deep copy.
* `v1` and `v2` have the different references of the different shallow lists.
* `v1` and `v2` have the different references of the different deep lists.
from copy import deepcopy
v1 = ['a', 'b', ['c', 'd']]
v2 = deepcopy(v1)
print(v1, v2) # ['a', 'b', ['c', 'd']] ['a', 'b', ['c', 'd']]
v2[1] = 'B'
v2[2][0] = 'C'
# ↓↓↓ ↓↓↓
print(v1, v2) # ['a', 'b', ['c', 'd']] ['a', 'B', ['C', 'd']]
*The below with copy() is equivalent to the above.
v1 = ['a', 'b', ['c', 'd']]
v2 = v1.copy()
v2[2] = v2[2].copy()
print(v1, v2) # ['a', 'b', ['c', 'd']] ['a', 'b', ['c', 'd']]
v2[1] = 'B'
v2[2][0] = 'C'
# ↓↓↓ ↓↓↓
print(v1, v2) # ['a', 'b', ['c', 'd']] ['a', 'B', ['C', 'd']]
*The below with copy() is equivalent to the above.
from copy import copy
v1 = ['a', 'b', ['c', 'd']]
v2 = copy(v1)
v2[2] = copy(v2[2])
print(v1, v2) # ['a', 'b', ['c', 'd']] ['a', 'b', ['c', 'd']]
v2[1] = 'B'
v2[2][0] = 'C'
# ↓↓↓ ↓↓↓
print(v1, v2) # ['a', 'b', ['c', 'd']] ['a', 'B', ['C', 'd']]
*The below with list() is equivalent to the above.
v1 = ['a', 'b', ['c', 'd']]
v2 = list(v1)
v2[2] = list(v2[2])
print(v1, v2) # ['a', 'b', ['c', 'd']] ['a', 'b', ['c', 'd']]
v2[1] = 'B'
v2[2][0] = 'C'
# ↓↓↓ ↓↓↓
print(v1, v2) # ['a', 'b', ['c', 'd']] ['a', 'B', ['C', 'd']]
*Additionally, the below is a 3D list.
from copy import deepcopy
v1 = ['a', 'b', ['c', ['d']]]
v2 = deepcopy(v1)
print(v1, v2) # ['a', 'b', ['c', ['d']]] ['a', 'b', ['c', ['d']]]
v2[1] = 'B'
v2[2][0] = 'C'
v2[2][1][0] = 'D'
# ↓↓↓ ↓↓↓ ↓↓↓
print(v1, v2) # ['a', 'b', ['c', ['d']]] ['a', 'B', ['C', ['D']]]
