What is shallow and deep copying in Python?

In Python, shallow copy and deep copy are two distinct methods for copying data, primarily used for complex data types such as lists and dictionaries. These copy methods are particularly important for handling nested data structures.

Shallow Copy

Shallow copy creates a new object but only copies the references from the original object (without copying the referenced objects themselves). This means that if the original data structure contains references to other objects, such as another list within a list, shallow copy will copy the reference to the internal list, not the internal list's content.

Example:

python
import copy

original_list = [1, 2, [3, 4]]
shallow_copied_list = copy.copy(original_list)

# Modify the nested list in the original list
original_list[2].append(5)

print(shallow_copied_list)  # Output: [1, 2, [3, 4, 5]]

In this example, modifying the nested list in the original list also affects the shallow copied list, as they share the same internal list object.

Deep Copy

Deep copy creates a new object and recursively copies all referenced objects. This means it copies all the content, not just the references, thereby avoiding dependencies between the original object and the copy.

Example:

python
import copy

original_list = [1, 2, [3, 4]]
deep_copied_list = copy.deepcopy(original_list)

# Modify the nested list in the original list
original_list[2].append(5)

print(deep_copied_list)  # Output: [1, 2, [3, 4]]

In this example, the deep copied list is not affected by modifications to the original list, as it is a completely independent copy.

Applicable Scenarios

• When the data structure is simple or does not contain nested structures, shallow copy is usually sufficient.
• When the data structure is complex, especially with multi-level nested structures, it is recommended to use deep copy to ensure data independence and avoid modifications to one data affecting another.

In summary, choosing between shallow copy and deep copy depends on the specific application scenario and requirements.