Copy Module - Shallow vs Deep Copy

Introduction

The copy module provides functions for copying Python objects. Understanding the difference between shallow and deep copies is crucial for avoiding subtle bugs.

Shallow Copy

import copy

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

shallow[1].append(5)
print(original)  # [1, [2, 3, 5], 4] - modified!
print(shallow)   # [1, [2, 3, 5], 4]

Deep Copy

import copy

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

deep[1].append(5)
print(original)  # [1, [2, 3], 4] - unchanged!
print(deep)      # [1, [2, 3, 5], 4]

Copying Custom Objects

import copy

class Node:
    def __init__(self, value, children=None):
        self.value = value
        self.children = children or []
    
    def __copy__(self):
        return Node(self.value, self.children)
    
    def __deepcopy__(self, memo):
        return Node(self.value, copy.deepcopy(self.children, memo))

node = Node(1, [Node(2), Node(3)])
copied = copy.deepcopy(node)

When to Use Each

import copy

# Shallow copy - for immutable data or flat structures
simple_list = [1, 2, 3]
shallow = copy.copy(simple_list)

# Deep copy - for nested mutable structures
nested = {"a": [1, 2], "b": [3, 4]}
deep = copy.deepcopy(nested)

Copy Module Functions

import copy

# copy() - shallow copy
shallow = copy.copy(obj)

# deepcopy() - deep copy
deep = copy.deepcopy(obj)

# copy.copy() and copy.deepcopy() equivalent

Practice Problems

1. Copy a nested dictionary correctly
2. Implement copy for a class
3. Handle circular references in deep copy
4. Copy a linked list structure
5. Create immutable copy semantics