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Openai·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say
May 2026Remote

Summary

Got a coding question at OpenAI that looked simple on the surface but had a few gotchas worth thinking through. The problem was self-contained but the carry logic and StopIteration boundary conditions are where people slip up.

Questions Asked (1)

Q1

Implement an IPv4 iterator class in Python. Given a starting IP address like '192.168.0.1', the class should iterate through every subsequent IPv4 address up to '255.255.255.255', following the standard Python iterator protocol with __init__, __iter__, and __next__.

Algorithms & Data StructuresAPI & IntegrationsTechnical Trade-offs
Author's notes

My first instinct was to convert the dotted-quad to a single integer and just increment it, which actually works cleanly.

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AI HintsAI Generated

Suggested Approach

Clarify the requirements and edge cases, then implement the iterator by converting the IP address to an integer, incrementing it, and converting back to dotted-decimal format. Ensure the iterator stops after yielding 255.255.255.255 and handles invalid inputs gracefully.

Pro tip: Mention that using integer conversion is more efficient than manipulating octets individually, and discuss the trade-offs between memory usage and simplicity. Also, consider thread safety if the iterator might be used concurrently.

1. Clarify Requirements and Edge Cases

Ask about expected behavior for invalid IPs, whether the starting IP is inclusive, and if the iterator should be reusable. Confirm that the iteration stops at 255.255.255.255.

2. Design the Iterator Class

Decide on internal representation: store the current IP as an integer for easy incrementing. Plan the __init__, __iter__, and __next__ methods, ensuring __iter__ returns self and __next__ raises StopIteration when exhausted.

3. Implement Conversion Helpers

Write helper functions to convert between dotted-decimal string and integer, handling validation. Use bitwise operations or arithmetic to pack/unpack octets.

4. Implement the Iterator Protocol

In __next__, check if current IP exceeds 255.255.255.255; if so, raise StopIteration. Otherwise, yield the current IP as a string, then increment the integer representation.

5. Test and Discuss Trade-offs

Test with edge cases like starting at 255.255.255.255, invalid inputs, and large ranges. Discuss time/space complexity and alternative approaches (e.g., using ipaddress module).

Key Points to Mention

  • Integer representation of IP addresses for efficient incrementing
  • Proper implementation of __iter__ and __next__ methods
  • Handling of StopIteration when reaching 255.255.255.255
  • Validation of input IP address format
  • Time and space complexity: O(1) per iteration, O(1) space
  • Trade-offs: manual conversion vs. using Python's ipaddress module

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