The Depth of Field

An easy Python interview practice problem on DataDriven. Write the Python and run it against test cases, with instant feedback.

Domain
Python
Difficulty
easy
Seniority
L3
Asked in
Citadel

Problem

A config export arrives as arbitrarily nested lists, but the consumer downstream only handles a fixed number of nesting levels. Collapse lst by exactly depth levels, preserving order, so a depth of 0 returns it untouched and anything still nested below the levels you collapse stays wrapped in its remaining sublists. A depth larger than the actual nesting simply flattens as far as the data goes.

Some containers hold containers that hold containers.

Example 1

Input:

lst = [[1,[2,[3]]]], depth = 1

Output:

[1,[2,[3]]]

Example 2

Input:

lst = [[1,[2,[3]]]], depth = 2

Output:

[1,2,[3]]

Worked solution and explanation

Why this problem exists in real interviews

Full flattening is the problem everyone has seen; the twist here is a brake. You are not collapsing everything, you are collapsing exactly depth levels and then stopping. The whole challenge lives in one decision: when you step into a sublist, do you remember that you just spent a level? Candidates who treat depth as a fixed constant instead of spending it on the way down either flatten everything or flatten nothing.

Break down the requirements

Step 1: Spend the budget, then stop

When you have no levels left to spend (depth is 0), every element is appended exactly as it is, even if it is itself a list. That is what keeps the deeper nesting intact.

Step 2: Descend only when you can afford it

For each element, only descend if it is a list AND you still have budget. Descend by recursing with depth decremented, so the brake is tracked separately on every branch of the nesting.

Step 3: Return the partially collapsed list

Elements nested deeper than the levels you collapsed come back wrapped in their remaining sublists, and the original left-to-right order is preserved throughout.

The solution

Depth-limited recursive flattening
def flatten_to_depth(lst: list, depth: int) -> list:
    result = []
    for item in lst:
        if isinstance(item, list) and depth > 0:
            result.extend(flatten_to_depth(item, depth - 1))
        else:
            result.append(item)
    return result

Common follow-up questions