What is the difference between a shallow copy and a deep copy?

Shallow copy:

Creates a new object but does not copy nested objects — it maintains references to the originals:

import copy

original = [[1, 2, 3], [4, 5, 6]]
shallow = copy.copy(original)

shallow[0][0] = 999
print(original[0][0])  # 999 — the original has changed too!

Deep copy:

Creates a completely independent clone, including all nested objects:

import copy

original = [[1, 2, 3], [4, 5, 6]]
deep = copy.deepcopy(original)

deep[0][0] = 999
print(original[0][0])  # 1 — the original remains unchanged

Ways to make a shallow copy:

# For lists
lst = [1, 2, 3]
copy1 = lst.copy()
copy2 = lst[:]
copy3 = list(lst)

# For dictionaries
d = {"a": 1}
copy4 = d.copy()
copy5 = dict(d)

# Universal
import copy
copy6 = copy.copy(lst)

When to use which:

• Shallow — when the collection only contains immutable elements (numbers, strings).
• Deep — when there are nested mutable objects (lists within lists, dictionaries within lists).

# Shallow is sufficient
nums = [1, 2, 3]  # Elements are immutable ints
safe_copy = nums.copy()

# Deep copy is required
matrix = [[1, 2], [3, 4]]  # Nested lists
safe_copy = copy.deepcopy(matrix)