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.

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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

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Lists and Tuples

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

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

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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]

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

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Q5. How do you copy a list correctly? Medium

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Q6. What is a list comprehension? Easy

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

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

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Q8. Are tuples completely immutable? Hard

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

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

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Sets and Dicts

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

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

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

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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}

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Q14. What is a dictionary in Python? Easy

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

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

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Q17. What is a dict comprehension? Medium

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

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Q18. Can a list be a dict key? Medium

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Collections Module

Q19. What is collections.Counter? Medium

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

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Q20. What is collections.defaultdict? Medium

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

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

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Q22. What is collections.namedtuple? Medium

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

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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)

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Complexity and Advanced

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

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

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

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

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

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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.

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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.

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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.

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Complexity Cheat Sheet

Operation	list	dict	set	deque
index / key access	O(1)	O(1) avg	n/a	O(1) ends
append	O(1) amortised	O(1) avg	O(1) avg	O(1)
pop front	O(n)	n/a	n/a	O(1)
membership (in)	O(n)	O(1) avg	O(1) avg	O(n)
insert middle	O(n)	n/a	n/a	O(n)

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Frequently Asked Questions

What data structure questions are most common in Python interviews in 2026?

Candidates report that the difference between list and tuple, how a dict and set achieve O(1) lookup, and when to use collections.deque or Counter are the most repeated Python data structure questions.

Do I need to know time complexity for Python built-ins?

Yes. Interviewers expect O(1) average for dict and set lookup, O(1) amortised for list append, and O(n) for list membership and front insert, which is exactly why deque is preferred for queues.

Is the collections module asked in interviews?

Frequently. Counter, defaultdict, deque, OrderedDict, and namedtuple all appear, with interviewers asking which one solves a given problem most cleanly.

Why can a tuple be a dict key but not a list?

Dict keys must be hashable, and hashability requires immutability. Tuples are immutable (assuming hashable contents) so they qualify, while lists are mutable and therefore unhashable.

When should I use a generator instead of a list?

Use a generator when the sequence is large or infinite and you process items one at a time, since it holds only the current state in memory rather than materialising the whole sequence.

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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

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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.