Even Filter

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

Problem

Given a list of integers, return a new list containing only the even numbers in their original order.

Only the even ones survive.

Example 1

Input:

nums = [1,2,3,4,5,6]

Output:

[2,4,6]

Example 2

Input:

nums = [1,3,5]

Output:

[]

Worked solution and explanation

Why this problem exists in real interviews

Filter-a-list-by-predicate is the warm-up interviewers open with to check Python fluency before the harder problem. The right answer is one expression; the wrong answer is a loop with an accumulator list and an if. What they actually grade is whether you reach for a list comprehension or filter(), whether you preserve input order, and whether you avoid mutating the input.

Break down the requirements

Step 1: Use a list comprehension with a predicate

[x for x in nums if x % 2 == 0] is the Pythonic one-liner. It preserves input order by construction (iteration is left to right), allocates the result once, and runs the predicate in the interpreter loop without the overhead of a named function.

Step 2: Use the modulo test, not bitwise tricks

x % 2 == 0 reads as English and handles negative integers and zero correctly (0 % 2 == 0, -2 % 2 == 0 in Python). Bitwise x & 1 == 0 also works but sacrifices readability for zero speedup on interview-sized inputs.

Step 3: Return a new list, never mutate the input

The spec says 'return a new list,' so produce a fresh object. A list comprehension already does this. Using nums.remove(x) in a loop would mutate and also be O(n^2) due to repeated shifts; both are wrong.

The solution

One-line comprehension
def even_filter(nums: list) -> list:
    return [x for x in nums if x % 2 == 0]

Common follow-up questions