5.17 Interview Questions

Frequently Asked Questions

• What is the difference between a list and a tuple?
• Why is bool considered a subclass of int?
• What’s the difference between == and is?
• Why can’t a list be used as a dictionary key, but a tuple can?
• What is the mutable default argument bug, and how do you fix it?
• What is the difference between type() and isinstance()?

Scenario-Based Questions

• You need to store a million IP addresses and check membership repeatedly and fast — which type do you choose, and why?
• A function silently accumulates data across unrelated calls — what bug pattern should you suspect first?
• Given a parsed JSON API response, what Python types will the JSON object, array, string, number, true/false, and null become?
• Why might 0.1 + 0.2 == 0.3 evaluate to False, and how would you compare floats safely instead?

Quick Interview Answer

“The data-type questions that come up most are: list vs tuple (mutability, hashability), why bool is a subclass of int, == vs is (value vs identity), why only hashable/immutable types work as dict keys, and the mutable default argument bug. Scenario questions usually test whether you’d reach for a set for fast membership checks, and whether you know JSON’s true/false/null map to bool/None in Python.”

Common Mistakes

• Answering “list and tuple are basically the same” without mentioning the mutability difference and its downstream consequences (hashability, dict-key eligibility).
• Not being able to explain why the mutable default argument bug happens (default values are evaluated once, at function definition time), only that it’s “a gotcha.”