Understanding Shallow Copy and Deep Copy in Python with Examples

When you need to duplicate an object, copying its reference is often insufficient; you must create an independent copy. This is where shallow copy and deep copy become essential.

Shallow Copy creates a new object and fills it with references to the original object's elements; immutable types are effectively duplicated, while mutable types retain references.

Deep Copy creates a new object and recursively copies all objects found within the original, producing a fully independent clone.

Example Code

First, import the copy module, which provides the copy and deepcopy functions.

1. Shallow copy of a list

import copy</code><code>original_list = [1, 2, [3, 4]]</code><code>shallow_copied_list = copy.copy(original_list)</code><code>original_list.append(5)</code><code>original_list[2].append(6)</code><code>print("Original List:", original_list)  # Output: [1, 2, [3, 4, 6], 5]</code><code>print("Shallow Copied List:", shallow_copied_list)  # Output: [1, 2, [3, 4, 6]]

2. Deep copy of a list

import copy</code><code>original_list = [1, 2, [3, 4]]</code><code>deep_copied_list = copy.deepcopy(original_list)</code><code>original_list.append(5)</code><code>original_list[2].append(6)</code><code>print("Original List:", original_list)  # Output: [1, 2, [3, 4, 6], 5]</code><code>print("Deep Copied List:", deep_copied_list)  # Output: [1, 2, [3, 4]]

3. Shallow copy of a dictionary

import copy</code><code>original_dict = {'a': 1, 'b': 2, 'c': [3, 4]}</code><code>shallow_copied_dict = copy.copy(original_dict)</code><code>original_dict['c'].append(5)</code><code>print("Original Dict:", original_dict)  # Output: {'a': 1, 'b': 2, 'c': [3, 4, 5]}</code><code>print("Shallow Copied Dict:", shallow_copied_dict)  # Output: {'a': 1, 'b': 2, 'c': [3, 4, 5]}

4. Deep copy of a dictionary

import copy</code><code>original_dict = {'a': 1, 'b': 2, 'c': [3, 4]}</code><code>deep_copied_dict = copy.deepcopy(original_dict)</code><code>original_dict['c'].append(5)</code><code>print("Original Dict:", original_dict)  # Output: {'a': 1, 'b': 2, 'c': [3, 4, 5]}</code><code>print("Deep Copied Dict:", deep_copied_dict)  # Output: {'a': 1, 'b': 2, 'c': [3, 4]}

5. Shallow copy of a custom class

import copy</code><code>class MyClass:</code><code>    def __init__(self, value):</code><code>        self.value = value</code><code>original_obj = MyClass([1, 2, 3])</code><code>shallow_copied_obj = copy.copy(original_obj)</code><code>original_obj.value.append(4)</code><code>print("Original Value:", original_obj.value)  # Output: [1, 2, 3, 4]</code><code>print("Shallow Copied Value:", shallow_copied_obj.value)  # Output: [1, 2, 3, 4]

6. Deep copy of a custom class

import copy</code><code>class MyClass:</code><code>    def __init__(self, value):</code><code>        self.value = value</code><code>original_obj = MyClass([1, 2, 3])</code><code>deep_copied_obj = copy.deepcopy(original_obj)</code><code>original_obj.value.append(4)</code><code>print("Original Value:", original_obj.value)  # Output: [1, 2, 3, 4]</code><code>print("Deep Copied Value:", deep_copied_obj.value)  # Output: [1, 2, 3]

7. Defining a custom __copy__ method

import copy</code><code>class MyClass:</code><code>    def __init__(self, value):</code><code>        self.value = value</code><code>    def __copy__(self):</code><code>        return MyClass(self.value[:])</code><code>original_obj = MyClass([1, 2, 3])</code><code>shallow_copied_obj = copy.copy(original_obj)</code><code>original_obj.value.append(4)</code><code>print("Original Value:", original_obj.value)  # Output: [1, 2, 3, 4]</code><code>print("Shallow Copied Value:", shallow_copied_obj.value)  # Output: [1, 2, 3]

8. Defining a custom __deepcopy__ method

import copy</code><code>class MyClass:</code><code>    def __init__(self, value):</code><code>        self.value = value</code><code>    def __deepcopy__(self, memo):</code><code>        return MyClass(copy.deepcopy(self.value, memo))</code><code>original_obj = MyClass([1, 2, 3])</code><code>deep_copied_obj = copy.deepcopy(original_obj)</code><code>original_obj.value.append(4)</code><code>print("Original Value:", original_obj.value)  # Output: [1, 2, 3, 4]</code><code>print("Deep Copied Value:", deep_copied_obj.value)  # Output: [1, 2, 3]

9. Shallow copy of a complex structure

import copy</code><code>original_complex = [1, 2, [3, 4], {'a': 5, 'b': [6, 7]}]</code><code>shallow_copied_complex = copy.copy(original_complex)</code><code>original_complex[2].append(8)</code><code>original_complex[3]['b'].append(9)</code><code>print("Original Complex:", original_complex)  # Output: [1, 2, [3, 4, 8], {'a': 5, 'b': [6, 7, 9]}]</code><code>print("Shallow Copied Complex:", shallow_copied_complex)  # Output: [1, 2, [3, 4, 8], {'a': 5, 'b': [6, 7, 9]}]

10. Deep copy of a complex structure

import copy</code><code>original_complex = [1, 2, [3, 4], {'a': 5, 'b': [6, 7]}]</code><code>deep_copied_complex = copy.deepcopy(original_complex)</code><code>original_complex[2].append(8)</code><code>original_complex[3]['b'].append(9)</code><code>print("Original Complex:", original_complex)  # Output: [1, 2, [3, 4, 8], {'a': 5, 'b': [6, 7, 9]}]</code><code>print("Deep Copied Complex:", deep_copied_complex)  # Output: [1, 2, [3, 4], {'a': 5, 'b': [6, 7]}]

These examples demonstrate that a shallow copy only duplicates the top‑level structure, while a deep copy recursively clones all nested objects, ensuring complete independence between the original and the copy.