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Openai·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say
Jun 2026Remote

Summary

Interviewed for a software engineering role at OpenAI and got a coding question that was basically a mini systems design problem disguised as a Python exercise. Not the most brutal interview I've had but it required more thought than I expected going in.

Questions Asked (1)

Q1

Build an in-memory key-value store in Python with set, get, serialize, and deserialize methods. It should handle type fidelity for common Python types and gracefully deal with missing keys or bad serialized input. Include basic tests showing data round-trips correctly.

System DesignTechnical Trade-offsAPI & Integrations
Author's notes

I went with JSON over pickle pretty quickly because pickle felt like a trap for a production-adjacent context (arbitrary code execution, version issues).

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and constraints, then design a simple class with a dictionary for storage and JSON-based serialization. Implement methods with robust error handling and type preservation, and write tests covering normal and edge cases. Discuss trade-offs and potential improvements.

Pro tip: Mention that while JSON is simple, it doesn't preserve all Python types (e.g., tuples become lists, sets aren't serializable). Propose using pickle for full type fidelity or a custom encoder/decoder, and discuss security implications of pickle.

1. Clarify Requirements and Constraints

Ask about expected data types, serialization format, performance needs, and error handling expectations. Confirm whether the store should be thread-safe or persistent.

2. Design the API and Data Model

Define the class interface: set(key, value), get(key), serialize(), deserialize(data). Choose an internal data structure (e.g., dict) and decide on serialization approach (e.g., JSON, pickle).

3. Implement Core Methods with Error Handling

Write set and get with checks for missing keys (return None or raise KeyError). Implement serialize to convert the store to a string/bytes, and deserialize to reconstruct it, handling invalid input gracefully.

4. Ensure Type Fidelity

Use a serialization method that preserves types (e.g., pickle) or implement custom encoding for common types (e.g., datetime, set). Document limitations if using JSON.

5. Write and Run Tests

Create tests for setting/getting values, serializing/deserializing, missing keys, and corrupt data. Verify round-trip fidelity for various types.

Key Points to Mention

  • Choice of serialization format (JSON vs pickle) and its impact on type fidelity and security
  • Error handling strategies: returning None, raising exceptions, or logging for missing keys and bad input
  • Thread safety considerations and potential use of locks
  • Performance implications of serialization and memory usage
  • Extensibility: supporting custom types via registration or subclassing
  • Testing edge cases: empty store, large data, non-string keys, and malformed serialized data

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.