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

Shallow copy creates a new object, but nested objects are shared. Deep copy creates copies of all nested objects.

1import copy
2
3original = [[1, 2], [3, 4]]
4
5# Shallow copy
6shallow = copy.copy(original)
7shallow[0][0] = 999
8print(original)  # [[999, 2], [3, 4]] — changed!
9
10# Deep copy
11original = [[1, 2], [3, 4]]
12deep = copy.deepcopy(original)
13deep[0][0] = 999
14print(original)  # [[1, 2], [3, 4]] — unchanged

Shallow copy methods:

• copy.copy().
• list.copy().
• list(original).
• Slicing original[:].

When to use:

• Shallow — nested objects are immutable or should be shared.
• Deep — complete independence of objects.