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

This distinction matters whenever you have nested or mutable objects and want an independent copy.

An assignment (b = a) creates a second name for the same object — not a copy at all. Mutating b mutates a.

A shallow copy creates a new container object but does not copy the objects inside it — the inner elements are still shared. You get a new list/dict/etc., but any mutable nested objects are referenced, not cloned.

A deep copy recursively copies every object, including nested ones, so the result is completely independent.

import copy

original = [[1, 2], [3, 4]]

# Shallow copy — new outer list, same inner lists
shallow = original.copy()      # or list(original) or original[:]
shallow[0].append(99)          # mutates the shared inner list!
print(original)  # [[1, 2, 99], [3, 4]]  — original changed

# Deep copy — new outer AND inner lists
original2 = [[1, 2], [3, 4]]
deep = copy.deepcopy(original2)
deep[0].append(99)
print(original2)  # [[1, 2], [3, 4]]  — original untouched

When to choose each:

• Shallow copy is sufficient when the container holds immutable values (ints, strings, tuples of immutables) or when you intentionally want the copy to share inner objects.
• Deep copy is needed when you want a fully independent snapshot — configuration trees, game states, undo stacks. It is slower and uses more memory.

For dicts, dict.copy() and {**original} are both shallow. The spread operator {**d} is commonly seen in interview code as a one-liner to create a modified copy of a dict without mutating the original.

Invest now in Acorns!!! 🚀
Join Acorns and get your $5 bonus!

Acorns is a micro-investing app that automatically invests your "spare change" from daily purchases into diversified, expert-built portfolios of ETFs. It is designed for beginners, allowing you to start investing with as little as $5. The service automates saving and investing. Disclosure: I may receive a referral bonus.

Invest now!!! Get Free equity stock (US, UK only)!

Use Robinhood app to invest in stocks. It is safe and secure. Use the Referral link to claim your free stock when you sign up!.

The Robinhood app makes it easy to trade stocks, crypto and more.

Webull! Receive free stock by signing up using the link: Webull signup.