Shallow Copy Vs Deep Copy in Python (Krish Naik)

Three Operation in Focus

• =
• copy()
• deepcopy()

1. = (Assignment ) - Same Reference

lst1 = [1, 2, 3, 4]
lst2 = lst1        # Not a copy, just another reference

lst2[1] = 1000     # Change through lst2
print(lst1)        # [1, 1000, 3, 4] — also changed

• lst2 = lst1 means both point to the same object.
• Any change through lst2 affects lst1.

2. copy() - Shallow Copy

lst1 = [1, 2, 3, 4]
lst2 = lst1.copy()   # Creates a new list

lst2[1] = 1000
print(lst1)          # [1, 2, 3, 4] — unchanged

• copy() creates a new list with same elements (primitives).
• So, changing one does not affect the other.

Shallow Copy with Nested Lists

lst1 = [[1, 2, 3, 4], [5, 6, 7, 8]]
lst2 = lst1.copy()

Case 1: Replace whole nested list

lst2[1] = [100, 200, 300, 400]
print(lst1)  # [[1, 2, 3, 4], [5, 6, 7, 8]] — unchanged

• Replacing whole inner list creates a new object → doesn’t affect original.

**Case 2: Append new nested list ⭐

lst1.append([2, 3, 4, 5])

print(lst1) # [[1, 2, 3, 4], [5, 6, 7, 8], [2, 3, 4, 5]]
print(lst2) # [[1, 2, 3, 4], [5, 6, 7, 8]]  — unchanged

• Adding new outer element → doesn’t affect original.

Case 3: Modify inner element

lst2[1][0] = 100
print(lst1)  # [[1, 2, 3, 4], [100, 6, 7, 8]] — changed!

• Because .copy() only copies the outer list, not the inner ones.
• Both lst1[1] and lst2[1] still refer to same inner list.

3. deepcopy() - Deep Copy

import copy

lst1 = [1, 2, 3, 4]
lst2 = copy.deepcopy(lst1)

lst2[1] = 100
print(lst1) # [1, 2, 3, 4] - unchanged

• In Non-Nested Object -> Shallow Copy = Deep Copy

Deep Copy with Nested Lists

lst1 = [[1, 2, 3], [3, 4, 5], [5, 6, 7]]
lst2 = copy.deepcopy(lst1)

lst2[1][0] = 100
print(lst1) = # [[1, 2, 3], [3, 4, 5], [5, 6, 7]] - unchanged

• deepcopy() creates new copies of all inner objects (lists inside list).
• So changes in lst2 won’t affect lst1, even in nested levels.
• This is different from .copy() which only copies the outer list and shares inner ones.