How to Keep a Variable Equal to Another Variable in Python

What will you learn?

You will learn how to ensure that one variable stays equal to another variable even after the second variable refers to a new mutable object.

Introduction to the Problem and Solution

In Python, when assigning one variable to another and then modifying the second variable by pointing it to a new mutable object, both variables can end up referencing the same memory location. This can lead to unexpected changes in the original variable. To prevent this issue and maintain the integrity of the first variable, creating a deep copy of the initial object before reassignment is necessary.

To tackle this challenge effectively, Python offers the copy module with its deepcopy() function. By leveraging deepcopy(), a complete replica of the original object is created. Subsequent alterations made on this copied object won’t impact the original variable, ensuring data integrity.


import copy

# Original list
A = [1, 2, 3]

# Making a deep copy of A into B
B = copy.deepcopy(A)

# Modifying B without affecting A

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

# Copyright PHD


In the provided code snippet: – The copy module is imported. – An original list A is defined. – The copy.deepcopy(A) method creates an independent clone of list A named B. – Changes made on list B do not reflect back on list A as they are distinct objects with separate memory references.

    How does shallow copying differ from deep copying?

    Shallow copying creates a new collection but references nested objects within it. Deep copying constructs entirely new compound objects and recursively inserts copies into it.

    Can I use slicing as an alternative for creating copies?

    Yes. Slicing is useful for creating shallow copies of lists quickly. For example:

    B = A[:]
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    Why should I prefer deepcopy over assignment or shallow copy?

    Deepcopy ensures complete independence between objects while assignment or shallow copies lead multiple variables referencing same memory locations causing unintentional updates across variables.

    Is deepcopy limited only for lists?

    No. The deepcopy() function can create duplicates of various data structures like dictionaries and sets besides lists.

    How does deepcopy handle recursive data structures?

    Deepcopy efficiently manages recursive data structures by tracking previously cloned elements using unique ids.

    Does deepcopy work with user-defined classes too?

    Yes. Deepcopy works with user-defined classes as long as they implement proper __deepcopy__() methods.


    Maintaining consistency between variables in Python is vital to prevent unintended side effects during program execution. By utilizing techniques like deep copying from the copy module, you can ensure that one variable remains unaffected by changes made to another variable referencing different mutable entities.

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