Shallow Copy & Deep Copy in Python (1)
#python#list#shallowcopy#deepcopy
Super Kai (Kazuya Ito)

Super Kai (Kazuya Ito) @hyperkai

About: I'm a web developer. Buy Me a Coffee: ko-fi.com/superkai SO: stackoverflow.com/users/3247006/super-kai-kazuya-ito X(Twitter): twitter.com/superkai_kazuya FB: facebook.com/superkai.kazuya

Joined:
Oct 21, 2021

Shallow Copy & Deep Copy in Python (1)

Publish Date: May 17
0 0

Buy Me a Coffee☕

*Memos:

  • My post explains the shallow and deep copy of a tuple.
  • My post explains the shallow copy of the set with a tuple.
  • My post explains the shallow and deep copy of the set with an iterator.
  • My post explains the shallow and deep copy of a dictionary.
  • My post explains the shallow and deep copy of an iterator.
  • My post explains a list and the list with indexing.

<Normal Copy>:

*Memos:

  • v1 and v2 refer to the same shallow and deep list.
  • is keyword can check if v1 and v2 refer to the same list. 
     #### Shallow list ####
#    ↓↓↓↓↓↓↓↓↓↓↓          ↓ 
v1 = ['a', 'b', ['c', 'd']]
v2 = v1       # ↑↑↑↑↑↑↑↑↑↑
              # Deep list

print(v1) # ['a', 'b', ['c', 'd']]
print(v2) # ['a', 'b', ['c', 'd']]

print(v1 is v2, v1[2] is v2[2])
# True True

v2[1] = 'B'
v2[2][0] = 'C'
          #       ↓↓↓   ↓↓↓
print(v1) # ['a', 'B', ['C', 'd']]
print(v2) # ['a', 'B', ['C', 'd']]
          #       ↑↑↑   ↑↑↑
Enter fullscreen mode Exit fullscreen mode

<Shallow Copy>:

list.copy() does shallow copy as shown below:

*Memos:

  • v1 and v2 refer to different shallow lists.
  • v1 and v2 refer to the same deep list. 
v1 = ['a', 'b', ['c', 'd']]
v2 = v1.copy()

print(v1) # ['a', 'b', ['c', 'd']]
print(v2) # ['a', 'b', ['c', 'd']]

print(v1 is v2, v1[2] is v2[2])
# False True

v2[1] = 'B'
v2[2][0] = 'C'
          #       ↓↓↓   ↓↓↓
print(v1) # ['a', 'b', ['C', 'd']]
print(v2) # ['a', 'B', ['C', 'd']]
          #       ↑↑↑   ↑↑↑
Enter fullscreen mode Exit fullscreen mode

The below with copy.copy() which does shallow copy is equivalent to the above:

import copy

v1 = ['a', 'b', ['c', 'd']]
v2 = copy.copy(v1)

print(v1) # ['a', 'b', ['c', 'd']]
print(v2) # ['a', 'b', ['c', 'd']]

print(v1 is v2, v1[2] is v2[2])
# False True

v2[1] = 'B'
v2[2][0] = 'C'
          #       ↓↓↓   ↓↓↓
print(v1) # ['a', 'b', ['C', 'd']]
print(v2) # ['a', 'B', ['C', 'd']]
          #       ↑↑↑   ↑↑↑
Enter fullscreen mode Exit fullscreen mode

The below with list() which does shallow copy is equivalent to the above:

v1 = ['a', 'b', ['c', 'd']]
v2 = list(v1)

print(v1) # ['a', 'b', ['c', 'd']]
print(v2) # ['a', 'b', ['c', 'd']]

print(v1 is v2, v1[2] is v2[2])
# False True

v2[1] = 'B'
v2[2][0] = 'C'
          #       ↓↓↓   ↓↓↓
print(v1) # ['a', 'b', ['C', 'd']]
print(v2) # ['a', 'B', ['C', 'd']]
          #       ↑↑↑   ↑↑↑
Enter fullscreen mode Exit fullscreen mode

The below with slicing which does shallow copy is equivalent to the above:

v1 = ['a', 'b', ['c', 'd']]
v2 = v1[:]

print(v1) # ['a', 'b', ['c', 'd']]
print(v2) # ['a', 'b', ['c', 'd']]

print(v1 is v2, v1[2] is v2[2])
# False True

v2[1] = 'B'
v2[2][0] = 'C'
          #       ↓↓↓   ↓↓↓
print(v1) # ['a', 'b', ['C', 'd']]
print(v2) # ['a', 'B', ['C', 'd']]
          #       ↑↑↑   ↑↑↑
Enter fullscreen mode Exit fullscreen mode

<Deep Copy>:

copy.deepcopy() does deep copy as shown below:

*Memos:

  • v1 and v2 refer to the different shallow and deep lists.
  • copy.deepcopy() should be used because it's safe, doing copy deeply while list.copy(), copy.copy(), list() and slicing aren't safe, doing copy shallowly. 
import copy 

v1 = ['a', 'b', ['c', 'd']]
v2 = copy.deepcopy(v1)

print(v1) # ['a', 'b', ['c', 'd']]
print(v2) # ['a', 'b', ['c', 'd']]

print(v1 is v2, v1[2] is v2[2])
# False False

v2[1] = 'B'
v2[2][0] = 'C'
          #       ↓↓↓   ↓↓↓
print(v1) # ['a', 'b', ['c', 'd']]
print(v2) # ['a', 'B', ['C', 'd']]
          #       ↑↑↑   ↑↑↑
Enter fullscreen mode Exit fullscreen mode

The below with list.copy() which does shallow copy is equivalent to the above:

v1 = ['a', 'b', ['c', 'd']]
v2 = v1.copy()
v2[2] = v1[2].copy()

print(v1) # ['a', 'b', ['c', 'd']]
print(v2) # ['a', 'b', ['c', 'd']]

print(v1 is v2, v1[2] is v2[2])
# False False

v2[1] = 'B'
v2[2][0] = 'C'
          #       ↓↓↓   ↓↓↓
print(v1) # ['a', 'b', ['c', 'd']]
print(v2) # ['a', 'B', ['C', 'd']]
          #       ↑↑↑   ↑↑↑
Enter fullscreen mode Exit fullscreen mode

The below with copy.copy() which does shallow copy is equivalent to the above:

import copy

v1 = ['a', 'b', ['c', 'd']]
v2 = copy(v1)
v2[2] = copy.copy(v1[2])

print(v1) # ['a', 'b', ['c', 'd']]
print(v2) # ['a', 'b', ['c', 'd']]

print(v1 is v2, v1[2] is v2[2])
# False False

v2[1] = 'B'
v2[2][0] = 'C'
          #       ↓↓↓   ↓↓↓
print(v1) # ['a', 'b', ['c', 'd']]
print(v2) # ['a', 'B', ['C', 'd']]
          #       ↑↑↑   ↑↑↑
Enter fullscreen mode Exit fullscreen mode

The below with list() which does shallow copy is equivalent to the above:

v1 = ['a', 'b', ['c', 'd']]
v2 = list(v1)
v2[2] = list(v1[2])

print(v1) # ['a', 'b', ['c', 'd']]
print(v2) # ['a', 'b', ['c', 'd']]

print(v1 is v2, v1[2] is v2[2])
# False False

v2[1] = 'B'
v2[2][0] = 'C'
          #       ↓↓↓   ↓↓↓
print(v1) # ['a', 'b', ['c', 'd']]
print(v2) # ['a', 'B', ['C', 'd']]
          #       ↑↑↑   ↑↑↑
Enter fullscreen mode Exit fullscreen mode

The below with slicing which does shallow copy is equivalent to the above:

v1 = ['a', 'b', ['c', 'd']]
v2 = v1[:]
v2[2] = v1[2][:]

print(v1) # ['a', 'b', ['c', 'd']]
print(v2) # ['a', 'b', ['c', 'd']]

print(v1 is v2, v1[2] is v2[2])
# False False

v2[1] = 'B'
v2[2][0] = 'C'
          #       ↓↓↓   ↓↓↓
print(v1) # ['a', 'b', ['C', 'd']]
print(v2) # ['a', 'B', ['C', 'd']]
          #       ↑↑↑   ↑↑↑
Enter fullscreen mode Exit fullscreen mode

Additionally, the below is a 3D list:

import copy 

v1 = ['a', 'b', ['c', ['d']]]
v2 = copy.deepcopy(v1)

print(v1) # ['a', 'b', ['c', ['d']]]
print(v2) # ['a', 'b', ['c', ['d']]]

print(v1 is v2, v1[2] is v2[2], v1[2][1] is v2[2][1])
# False False False

v2[1] = 'B'
v2[2][0] = 'C'
v2[2][1][0] = 'D'
          #       ↓↓↓   ↓↓↓   ↓↓↓
print(v1) # ['a', 'b', ['c', ['d']]]
print(v2) # ['a', 'B', ['C', ['D']]]
          #       ↑↑↑   ↑↑↑   ↑↑↑
Enter fullscreen mode Exit fullscreen mode

Comments 0 total

    Add comment