Python Data Structures Interview Questions 2026: 28 Q&A

Last Updated: June 2026 | Level: Freshers to Mid-Level | Read Time: ~16 min

Python's built-in data structures are interview gold because picking the right one signals real engineering judgement. Candidates report that list versus tuple, dict and set internals, and the collections module are guaranteed topics. This guide covers 28 questions with full answers, complexity guarantees, and predict-the-output traps. Behaviour reflects the official Python standard library documentation; confirm any complexity claim on the official docs.

Pair this with Python Interview Questions 2026 and Python List Dict Tuple Interview Questions 2026.

Table of Contents

1. Lists and Tuples (Q1 to Q9)
2. Sets and Dicts (Q10 to Q18)
3. Collections Module (Q19 to Q24)
4. Complexity and Advanced (Q25 to Q28)
5. Predict the Output
6. Complexity Cheat Sheet
7. Frequently Asked Questions

Lists and Tuples

Q1. What is the difference between a list and a tuple? Easy

Q2. When would you choose a tuple over a list? Medium

Q3. What is list slicing? Easy

a = [0, 1, 2, 3, 4]
print(a[1:4])     # [1, 2, 3]
print(a[::-1])    # [4, 3, 2, 1, 0]

Q4. What is the difference between append and extend? Easy

Q5. How do you copy a list correctly? Medium

Q6. What is a list comprehension? Easy

squares = [x*x for x in range(5) if x % 2 == 0]  # [0, 4, 16]

Q7. What is the difference between sort and sorted? Medium

Q8. Are tuples completely immutable? Hard

t = (1, [2, 3])
t[1].append(4)     # allowed, t is now (1, [2, 3, 4])

Q9. How do you unpack a tuple or list? Easy

Sets and Dicts

Q10. What is a set and what is it good for? Easy

Q11. How does a set achieve O(1) lookup? Medium

Q12. What is the difference between a set and a frozenset? Medium

Q13. What set operations are supported? Easy

a, b = {1, 2, 3}, {2, 3, 4}
print(a & b)   # {2, 3}
print(a | b)   # {1, 2, 3, 4}

Q14. What is a dictionary in Python? Easy

Q15. How does a Python dict maintain insertion order? Hard

Q16. How do you safely access a missing dict key? Medium

Q17. What is a dict comprehension? Medium

inv = {v: k for k, v in {"a": 1, "b": 2}.items()}  # {1: 'a', 2: 'b'}

Q18. Can a list be a dict key? Medium

Collections Module

Q19. What is collections.Counter? Medium

from collections import Counter
print(Counter("banana"))          # Counter({'a': 3, 'n': 2, 'b': 1})

Q20. What is collections.defaultdict? Medium

from collections import defaultdict
groups = defaultdict(list)
groups["a"].append(1)   # no KeyError

Q21. What is collections.deque and why use it for queues? Medium

Q22. What is collections.namedtuple? Medium

Q23. What is collections.OrderedDict still good for? Hard

Q24. How do you implement an LRU cache in Python? Hard

from functools import lru_cache
@lru_cache(maxsize=128)
def fib(n): return n if n < 2 else fib(n-1) + fib(n-2)

Complexity and Advanced

Q25. What is the time complexity of common list operations? Medium

Q26. Why is x in a_list slow but x in a_set fast? Medium

Q27. What is a generator and how does it save memory? Medium

def squares(n):
    for i in range(n): yield i * i

Q28. What is the difference between a shallow and a deep copy? Hard

Predict the Output

Snippet 1

a = [1, 2, 3]
b = a
b.append(4)
print(a)

Output:

[1, 2, 3, 4]

Why: b = a aliases the same list, so appending through b is visible through a. Use a.copy() for an independent list.

Snippet 2

print([1, 2, 3] * 2)
print({1, 2} | {2, 3})

Output:

[1, 2, 3, 1, 2, 3]
{1, 2, 3}

Why: * repeats a list; | unions sets, de-duplicating the shared element.

Snippet 3

d = {"a": 1}
print(d.get("b", 0))
print("b" in d)

Output:

0
False

Why: get returns the default 0 without inserting, so the key "b" still does not exist.

Complexity Cheat Sheet

Related Articles

• Python Interview Questions 2026, the master set
• Python List Dict Tuple Interview Questions 2026, built-in containers deep dive
• Python OOPs Interview Questions 2026, object model
• Data Structures Interview Questions 2026, language-agnostic DSA

Confirm any complexity guarantee on the official Python documentation before your interview. This guide reflects candidate-reported patterns and public preparation resources as of June 2026.