Top 10 Trace Table Questions (With Answers) - Interview Ready

This article collects ten small trace problems frequently seen in interviews. Each includes a short explanation so you can check your mental model quickly.

1) Mutable default accumulator:
def add(x, lst=[]):
    lst.append(x)
    return lst
Explain why multiple calls accumulate values and how to fix it.

Answer: default list is created at function definition time and reused across calls. Fix by using `None` and creating a new list inside the function when needed.

2) Late-binding lambda in loop:
funcs = [lambda: i for i in range(3)]
print([f() for f in funcs]) - what prints?

Answer: [2,2,2] because lambdas capture the variable `i` by reference; evaluate when the function runs. Fix by capturing the value with a default param: `lambda i=i: i`.

3) Generator exhaustion: calling next() after end raises StopIteration - always consider generator state in your trace.

4) Comprehension scope: in Python 3, comprehension variables do not leak to the outer scope - trace them as separate temporary frames.

5) Exception flow: when an exception occurs, stop normal lines and unwind frames until a matching handler is found. The assignment that raised does not complete.

6) Mutable aliasing (see example above).

7) Chained comparisons evaluate left-to-right with short-circuit semantics. '1 < 2 < 3' is True but be careful with mixed operators.

8) Dictionary mutation during iteration raises a RuntimeError. Copy keys to mutate safely: `for k in list(d.keys()):`.

9) Decorator wrapping: when tracing a decorated function, trace both wrapper and wrapped function - the wrapper runs first and may transform inputs/outputs.

10) Recursion base case mistakes: always mark base cases in your trace to avoid infinite recursive narration mistakes.

Practice these ten problems weekly - they cover the highest-return concepts interviewers test with short trace puzzles.