L2 · Working engineerObject-oriented Python~5 min · 3 tests#80

Composition vs Inheritance: Car Has an Engine

Model 'a Car has an Engine' with composition instead of inheritance, delegating start() to the engine. Includes dependency injection. Tested live.

The problem

Engine has running = False and start() that sets running = True and returns "Engine started".
Car(engine) has an engine (composition — Car does not inherit from Engine). Car.start() starts its engine, then returns "Car started".

Examples

Example 1

Input

probe(Car, Engine)

Expected output

('Car started', True, False)

Example 2
Input
```
Engine().start()
```
Expected output
```
'Engine started'
```

+ 1 hidden test on Submit — any engine can be injected.

Edge cases to ask about

Injected fake engine
Car is not an Engine

How the tests call your code

These helpers run before your code. The test inputs above call them.

def probe(Car, Engine):
    e = Engine()
    c = Car(e)
    msg = c.start()
    return (msg, e.running, isinstance(c, Engine))

class FakeEngine:
    def __init__(self):
        self.calls = 0
    def start(self):
        self.calls += 1
        return "fake"

def with_fake(Car):
    f = FakeEngine()
    Car(f).start()
    return f.calls

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 · Car

class Engine:
    def __init__(self):
        self.running = False

    def start(self):
        pass


class Car:
    def __init__(self, engine):
        pass

    def start(self):
        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
Composition + delegation	O(1)	O(1)	Pass the engine in (dependency injection) so tests can swap it.

Walkthrough of the optimal approach (try it yourself first)

Inheritance says is-a; composition says has-a. A Car has an Engine, so it holds one and delegates to it. Passing the engine into the constructor (dependency injection) means a test can hand in a fake engine — the hidden test does exactly that.

"Favour composition over inheritance" is the design principle interviewers want named.

Complexity: O(1) time, O(1) space. One delegated call.

Reveal the reference solution

class Engine:
    def __init__(self):
        self.running = False

    def start(self):
        self.running = True
        return "Engine started"


class Car:
    def __init__(self, engine):
        self.engine = engine

    def start(self):
        self.engine.start()
        return "Car started"

Follow-ups interviewers ask

Add an ElectricEngine without changing Car.
What breaks if Car inherited from Engine?

Frequently asked interview questions

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

What is the time complexity of Composition vs Inheritance: Car Has an Engine in Python?

The optimal solution runs in O(1) time and O(1) auxiliary space. One delegated call.

What follow-up questions do interviewers ask about Composition vs Inheritance: Car Has an Engine?

Add an ElectricEngine without changing Car. What breaks if Car inherited from Engine?