Copy vs Deep Copy

Copying is easy when data is flat, but tricky when nested.

Shallow copy vs deep copy

• Shallow copy copies the outer container only
• Deep copy copies everything inside (nested items too)

Shallow copy example (problem)

a = [[1, 2], [3, 4]]
b = a.copy()
b[0].append(99)

print(a)  # [[1,2,99],[3,4]] <-- changed!
print(b)

Why?
Because inner lists are still shared.

Deep copy (solution)

import copy

a = [[1, 2], [3, 4]]
b = copy.deepcopy(a)
b[0].append(99)

print(a)  # [[1,2],[3,4]]
print(b)  # [[1,2,99],[3,4]]

Graph: shallow vs deep

flowchart LR
  A[Outer list] --> B[Inner list 1]
  A --> C[Inner list 2]
  D[Shallow copy] --> B
  D --> C
  E[Deep copy] --> F[New inner list 1]
  E --> G[New inner list 2]

Remember

• Shallow copy is fine for flat structures
• Nested structures often need deepcopy
• Know this to avoid hidden bugs