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

In Python, a shallow copy and a deep copy are two different approaches to copying objects, including lists, dictionaries, and other mutable data structures. The main difference between a shallow copy and a deep copy lies in how they handle nested objects or references within the copied data structure.

Shallow Copy:

  • A shallow copy creates a new object but maintains references to the nested objects.
  • The top-level structure is copied, but the internal objects are not duplicated.
  • If you modify a nested object in the original or the shallow copy, the change will be reflected in both.
  • Shallow copying is performed using the copy() method or the [:] slice notation.
import copy

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

# Modifying the nested object in the original or shallow copy affects both
original[1].append(4)
print(original)        # Output: [1, [2, 3, 4]]
print(shallow_copy)    # Output: [1, [2, 3, 4]]

Deep Copy:

  • A deep copy creates a new object and recursively copies all the nested objects.
  • Both the top-level and internal objects are duplicated, resulting in independent copies.
  • Modifications to the nested objects in the original or the deep copy do not affect each other.
  • Deep copying is performed using the deepcopy() function from the copy module.
import copy

original = [1, [2, 3]]
deep_copy = copy.deepcopy(original)

# Modifying the nested object in the original doesn't affect the deep copy
original[1].append(4)
print(original)        # Output: [1, [2, 3, 4]]
print(deep_copy)       # Output: [1, [2, 3]]

By understanding the distinction between shallow copy and deep copy, you can choose the appropriate copying method based on your requirements. If you need to create independent copies of nested objects, a deep copy is necessary. If you want to maintain references to the original nested objects, a shallow copy suffices.

It's worth noting that deep copying can be more computationally expensive and may not be needed in all scenarios. Therefore, it's important to consider the complexity of the data structure and the desired behavior when deciding between shallow copy and deep copy.

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