Wrong API usage interview questions

Question 1

add_tag() should return a fresh tag list for each post unless one is passed in. Why do tags from one post show up on another?

def add_tag(tag: str, tags: list[str] = []) -> list[str]:
    """Append a tag and return the list of tags."""
    if tag not in tags:
        tags.append(tag)
    return tags

first = add_tag("python")   # ["python"]
second = add_tag("sql")     # expected ["sql"]

Accepted Answer

Line 1: Default values are evaluated once, when the function is defined, so every call without `tags` shares the same list. `second` comes back as ["python", "sql"], and the list keeps growing for the life of the process.

Question 2

topScores() should return the three highest scores. What does it return for [100, 25, 9, 1000]?

function topScores(scores) {
  return [...scores].sort().reverse().slice(0, 3);
}

Accepted Answer

Line 2: Without a comparator, sort() converts values to strings and sorts them lexicographically: [100, 1000, 25, 9]. Reversed, that gives [9, 25, 1000], and 100 is dropped.

Question 3

isAdmin() gates the admin dashboard. Why are real admins being locked out?

public boolean isAdmin(HttpServletRequest request) {
    String role = request.getHeader("X-Role");
    return role == "admin";
}

Accepted Answer

Line 3: `==` compares object references, not contents. The header value is a new String built at runtime, so it is never the same object as the "admin" literal and the check is always false.

Wrong API usage: interview questions

How to spot it in code review

Why AI-generated code gets this wrong

Wrong API usage: 3 practice questions

Mutable default argument

Sorting numbers with sort()

Comparing strings with ==

Attempts distribution