Dictionary Interview Questions
3 min read ·
Below are important FAANG-level interview questions on Python dictionaries, focusing on concepts, edge cases, and real-world usage.
1. What Is a Dictionary in Python?
A dictionary is a mutable, key–value data structure with O(1) average lookup time.
2. Why Are Dictionary Lookups Fast?
Because dictionaries use a hash table internally.
- Average time complexity: O(1)
- Worst case: O(n) (rare)
3. Can a Dictionary Have Duplicate Keys?
No.
If you repeat a key, the last value overwrites the previous one.
4. What Types Can Be Used as Dictionary Keys?
Keys must be immutable and hashable.
Valid:
Invalid:
5. Difference Between get() and []
get() is safer in production code.6. Difference Between pop() and del
7. What Does popitem() Do?
Removes and returns the last inserted key–value pair (Python 3.7+).
8. How to Merge Two Dictionaries?
9. Shallow Copy vs Deep Copy (Tricky)
Inner dictionary is shared.
Deep copy:
10. How to Count Frequency Using Dictionary?
11. How to Sort a Dictionary by Value?
Returns a list of tuples.
12. Are Dictionaries Ordered?
Yes, insertion order is preserved from Python 3.7+.
13. Can Dictionaries Be Used in Multithreading?
They are not thread-safe by default.
Use locks or concurrent data structures.
14. How to Invert a Dictionary?
Values must be unique.
15. Dictionary vs JSON (Interview Trap)
- Dictionary → Python object
- JSON → Text format
Conversion:
16. Time Complexity of Common Operations
| Operation | Complexity |
|---|---|
| Lookup | O(1) |
| Insert | O(1) |
| Delete | O(1) |
| Iteration | O(n) |
17. Can Dictionary Keys Change?
No.
Keys are immutable and cannot be modified after insertion.
18. What Is defaultdict?
Automatically initializes missing keys.
Used heavily in FAANG coding rounds.
19. How to Group Data Using Dictionary?
20. Real FAANG Insight
In real systems, dictionaries are used as in-memory indexes (similar to database indexes).
Understanding hashing, collisions, and immutability is more important than memorizing methods.
This mindset separates interview-level coders from production-ready engineers.