The Shadow Cleaner

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
Natera

Problem

Return a new list of items with every repeat after a value's first appearance dropped, the way we clean the ID columns in our event exports. Write the cleanup yourself: a one-call helper that collapses the whole list at once, such as list(set(items)) or dict.fromkeys(items), is off limits, though keeping your own record of values already seen is fine.

Remove the repeats. No shortcuts.

Example

Input:

items = [3,1,4,1,5,9,2,6,5,3]

Output:

[3,1,4,5,9,2,6]

Worked solution and explanation

Why this problem exists in real interviews

This tests order-preserving deduplication without built-in shortcuts. It probes whether a candidate understands how to track seen elements manually using a set for O(1) lookups while preserving insertion order with a list.

Break down the requirements

Step 1: Track which elements have been seen

Use a set (built incrementally, not via set() constructor on the full list) to record elements as you encounter them.

Step 2: Iterate and collect first occurrences

For each element, if it has not been seen, add it to both the result list and the seen set.

The solution

Manual seen-set with order preservation
def dedup(items: list) -> list:
    seen = set()
    result = []
    for item in items:
        if item not in seen:
            seen.add(item)
            result.append(item)
    return result

Common follow-up questions