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Debugging Under Pressure

How to fix your code when the interviewer says "I don't think that's right."

Introduction

In a real interview, you might hit "Run" and get a red Exception or Wrong Answer. Your heart rate spikes. You have 5 minutes left.

How you react in this exact moment often determines if you get the offer or the rejection.

1. Do Not Panic

The worst thing you can do is start randomly changing variables (i + 1 to i - 1) hoping it works. This is called "shotgun debugging" and it screams junior engineer.

Take a deep breath. Say out loud: "Okay, we have a bug. Let me trace the execution to see where the state diverges from my expectation."

2. Read the Error Message

If it's an exception (like IndexError: list index out of range), read the line number. Go straight to that line. Ask yourself: "Under what condition does this index exceed the length of the list?"

Common Python Exceptions in interviews: - IndexError: Loop went too far, or you forgot to check if not nums. - KeyError: You tried to access my_dict[val] without checking if val in my_dict. Use .get() or defaultdict. - RecursionError: You forgot a base case, or Python's 1000 limit was hit. - TypeError: 'NoneType' object is not subscriptable: A function returned None when you expected an array, usually because you forgot a return statement in a recursive call.

3. Print Debugging

If the platform allows you to run code, use print() statements aggressively. But be systematic.

Do not just print the variable. 

print(i) # BAD: What is this?
print(f"Index i={i}, Current Value={nums[i]}") # GOOD: Explicit context

Place prints at the entry of loops, the exit of loops, and right before the return statement. This will narrow down exactly which block of code is failing.

4. The Interviewer's Hint

If the interviewer says: "Are you sure about line 45?"

STOP. They are literally handing you the answer. Do not argue. Do not say "Yes, because..."

Immediately look at line 45 and assume it is completely wrong. Say: "Let me double-check line 45. Ah, I see, I'm using <= instead of <. Thank you for pointing that out."

Key Takeaways

  • Never shotgun debug.
  • Read the explicit error trace and line number.
  • Use structured print() statements to track state.
  • If the interviewer gives a hint, accept it immediately and act on it.