The Gap Filler

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
L4
Asked in
Meta

Problem

Given a list that may contain None values, return a list of the same length where each None is replaced by the most recent prior non-None value. If None appears before any non-None value, leave it as None.

Fill the Nones with the last real value.

Example 1

Input:

values = [1,null,2,3,null,null,5,null]

Output:

[1,1,2,3,3,3,5,5]

Example 2

Input:

values = [null,null,3,null]

Output:

[null,null,3,3]

Worked solution and explanation

Why this problem exists in real interviews

This tests forward fill, one of the most common time-series imputation techniques. It probes single-pass state tracking and correct handling of edge cases at the boundaries.

Break down the requirements

Step 1: Track the last non-None value

Initialize to None. Update whenever a non-None value is encountered.

Step 2: Replace None with the tracked value

If the tracked value is still None (no prior non-None seen), the gap stays None.

The solution

Forward fill with last-known-value tracking
def forward_fill(values: list) -> list:
    result = []
    last_known = None
    for val in values:
        if val is not None:
            last_known = val
        result.append(last_known)
    return result

Common follow-up questions