L2 · Working engineerObject-oriented Python~6 min · 4 tests#75

Abstract Base Class: Shape, Circle, Rectangle

Use Python's abc module to define an abstract Shape with area(), then implement Circle and Rectangle. Instantiating Shape must fail. Tested live.

The problem

Shape is an abstract base class with an abstract method area(). Shape() must raise TypeError.
Circle(radius) and Rectangle(width, height) implement area().

Examples

Example 1
Input
```
round(Circle(2).area(), 4)
```
Expected output
```
12.5664
```
Example 2
Input
```
Rectangle(3, 4).area()
```
Expected output
```
12
```

+ 2 hidden tests on Submit — abstract class cannot be created.

Edge cases to ask about

Instantiating the abstract class
Float rounding

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 · Shape

from abc import ABC, abstractmethod
import math


class Shape:
    def area(self):
        pass


class Circle(Shape):
    pass


class Rectangle(Shape):
    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
ABC + @abstractmethod	O(1)	O(1)	Python refuses to instantiate a class with unimplemented abstract methods.

Walkthrough of the optimal approach (try it yourself first)

Inherit from ABC and decorate area with @abstractmethod. Python then refuses to instantiate Shape (or any subclass that does not implement area) with a TypeError. That turns a missing method from a runtime surprise into an immediate error.

typing.Protocol is the structural ("duck-typed") alternative.

Complexity: O(1) time, O(1) space. Each area() is a constant-time formula.

Reveal the reference solution

from abc import ABC, abstractmethod
import math


class Shape(ABC):
    @abstractmethod
    def area(self):
        ...


class Circle(Shape):
    def __init__(self, radius):
        self.radius = radius

    def area(self):
        return math.pi * self.radius ** 2


class Rectangle(Shape):
    def __init__(self, width, height):
        self.width = width
        self.height = height

    def area(self):
        return self.width * self.height

Follow-ups interviewers ask

Add an abstract property perimeter.
ABC vs Protocol?

Frequently asked interview questions

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

What is the time complexity of Abstract Base Class: Shape, Circle, Rectangle in Python?

The optimal solution runs in O(1) time and O(1) auxiliary space. Each area() is a constant-time formula.

What follow-up questions do interviewers ask about Abstract Base Class: Shape, Circle, Rectangle?

Add an abstract property perimeter. ABC vs Protocol?