L2 · Working engineerPython internals~2 min · 4 tests#55

List Comprehension: Filter Even Numbers

Filter even numbers with a Python list comprehension, and compare it with a for-loop and filter(). Learn comprehension syntax with live tests.

The problem

Return the even numbers from nums using a single list comprehension.

Examples

Example 1
Input
```
evens([1, 2, 3, 4, 5, 6])
```
Expected output
```
[2, 4, 6]
```
Example 2
Input
```
evens([1, 3])
```
Expected output
```
[]
```

+ 2 hidden tests on Submit — 0 and negatives.

Edge cases to ask about

Zero
Negative numbers
Empty

Hints

0/3

How an interviewer scores this

0/9

1. Understand the problemRestate it, ask about edge cases and input size.
2. Write a brute-force solutionCorrect first, clever later.
3. Get the correct outputautoAll visible examples pass.
4. Analyse time complexityautoCommit to a Big-O in the Complexity Lab.
5. Analyse space complexityautoExtra memory, not the input.
6. Identify the bottleneckautoMeasure your code — where does it bend?
7. OptimiseUsually: a set or dict instead of a scan.
8. Explain why it worksSay it out loud to Solvi.
9. Test the edge casesautoSubmit passes every hidden test.

Python 3.13 · evens

def evens(nums):
    """Even numbers, via one list comprehension."""
    # Write your solution here
    pass

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Complexity Lab

What does this cost as n grows?

Interviewers score the analysis as much as the code. Commit to an answer first — then check it, and read why.

Time complexity of the optimal solution

Space complexity (extra memory)

Pick both to reveal the answer.

From brute force to optimal

The progression an interviewer wants to hear, one step at a time.

Approach	Time	Space	Idea
List comprehension	O(n)	O(n)	[expr for x in it if cond]

Walkthrough of the optimal approach (try it yourself first)

[x for x in nums if x % 2 == 0]. A comprehension is not just shorter — it avoids the repeated .append attribute lookup, so it is usually faster than the equivalent loop. Use a generator expression (x for …) when you only need to iterate once.

Complexity: O(n) time, O(n) space. One pass; the result can hold every element.

Reveal the reference solution

def evens(nums):
    return [x for x in nums if x % 2 == 0]

Follow-ups interviewers ask

Square the evens in the same comprehension.
When would you use a generator expression instead?

Frequently asked interview questions

Core interview concepts, complexities, and follow-ups scored by hiring teams.

What is the time complexity of List Comprehension: Filter Even Numbers in Python?

The optimal solution runs in O(n) time and O(n) auxiliary space. One pass; the result can hold every element.

What follow-up questions do interviewers ask about List Comprehension: Filter Even Numbers?

Square the evens in the same comprehension. When would you use a generator expression instead?