L2 · Working engineerPython internals~5 min · 5 tests#59

Fibonacci Generator With yield

Write a Python generator that yields the first n Fibonacci numbers lazily with yield, using O(1) memory. Learn how generators differ from lists.

The problem

Write a generator function fib_gen(n) that yields the first n Fibonacci numbers: 0, 1, 1, 2, 3, 5, ….

It must be a generator (use yield), not a function that returns a list.

Examples

Example 1
Input
```
list(fib_gen(7))
```
Expected output
```
[0, 1, 1, 2, 3, 5, 8]
```
Example 2
Input
```
type(fib_gen(3)).__name__
```
Expected output
```
'generator'
```

+ 3 hidden tests on Submit — lazy: values on demand.

Edge cases to ask about

n = 0
n = 1
Partial consumption with next()

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 · fib_gen

def fib_gen(n):
    """Yield the first n Fibonacci numbers."""
    # Write your solution here
    pass

⌘/Ctrl + Enter runs the examples

Your code runs in real CPython inside your browser — nothing is sent anywhere. The first run downloads the interpreter (about 6 MB, once). Your code is saved on this device as you type.

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
Build a list	O(n)	O(n)	Holds every value at once.
bestGenerator with yield	O(n)	O(1)	Produces one value at a time, on demand.

Walkthrough of the optimal approach (try it yourself first)

A function with yield returns a generator: calling it runs nothing; each next() resumes the function until the next yield. Only a and b live in memory, so fib_gen(10**9) costs nothing until you iterate.

Generators can be consumed once — a second list(g) is empty.

Complexity: O(n) time, O(1) space. Each value costs O(1) to produce and only two numbers are kept in memory, however large n is.

Reveal the reference solution

def fib_gen(n):
    a, b = 0, 1
    for _ in range(n):
        yield a
        a, b = b, a + b

Follow-ups interviewers ask

An infinite generator plus itertools.islice.
What does yield from do?

Frequently asked interview questions

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

What is the time complexity of Fibonacci Generator With yield in Python?

The optimal solution runs in O(n) time and O(1) auxiliary space. Each value costs O(1) to produce and only two numbers are kept in memory, however large n is.

What is the brute-force approach, and how do you optimise it?

Build a list: O(n) time, O(n) space. Holds every value at once. Generator with yield: O(n) time, O(1) space. Produces one value at a time, on demand.

What follow-up questions do interviewers ask about Fibonacci Generator With yield?

An infinite generator plus itertools.islice. What does yield from do?