What is difference between copy() and deepcopy() in Python

[fastqa]

In Python, copy and deepcopy come from the copy module and are used to duplicate objects.

The key difference lies in how they handle nested objects.

1. Shallow Copy (copy.copy())

Creates a new object but does not recursively copy nested objects (like lists or dicts inside the object).

If the original object contains mutable objects (lists, dicts, etc.), modifications to those mutables in the copied object will affect the original.

import copy

original = [1, [2, 3], 4]
shallow_copy = copy.copy(original)

shallow_copy[1][0] = 99  # Modifies the nested list
print(original)  # Output: [1, [99, 3], 4]

Explanation: Since copy.copy() only creates a new top-level object but keeps references to inner objects, changes in the nested list shallow_copy[1] reflect in the original.

2. Deep Copy (copy.deepcopy())

Creates a new object and recursively copies all nested objects.
Modifications in the copied object will not affect the original.

deep_copy = copy.deepcopy(original)

deep_copy[1][0] = 42
print(original)  # Output: [1, [99, 3], 4] (unchanged)

Explanation: deepcopy() creates an entirely independent copy of the object, including all nested elements.

3. When to Use?

Use copy.copy() when your object only contains immutable elements (like numbers, strings, tuples) or when shallow copying is sufficient.

Use copy.deepcopy() when your object contains nested mutable objects and you want a fully independent copy.