Shape Classes with Area Method

This was one of the two questions I got when I took the test. It is a basic classes question: two constructors and two area methods.

What the question asks

Write two classes:

• Rectangle, created with the lengths of its two sides. Its area() method returns the product of the two sides.
• Circle, created with its radius. Its area() method returns pi times the radius squared. The question asks for an accurate value of pi, and the easy way to get one is math.pi.

HackerRank’s code reads a number of queries. Each query is a line such as rectangle 2 3 or circle 5. It builds the shape, calls area() and prints the result with two decimal places.

Function to write

class Rectangle:
    def __init__(self, length, width): ...
    def area(self): ...

class Circle:
    def __init__(self, radius): ...
    def area(self): ...

You only write the two classes. The stub already has import math at the top and handles reading input and printing.

Solution

import math


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

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


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

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

The import math line is already in HackerRank’s stub. I keep it in my copy so the code also runs on its own.

How it works

• __init__ runs when the stub writes Rectangle(2, 3). It stores both sides on the object as self.length and self.width.
• area() reads those stored values and multiplies them.
• Circle stores the radius the same way. self.radius ** 2 squares it, and math.pi is pi to 15 decimal places.
• The two classes do not need a common parent class. The stub only calls area() on whatever shape it built.

Mistakes that fail the hidden tests

• Using 3.14 for pi. For radius 5, 3.14 * 25 prints 78.50, but the expected answer is 78.54. Small radii pass and larger ones fail, which is confusing if you do not know why.
• Single underscores in _init_. It must be __init__ with two underscores on each side. With _init_, Python never calls it, and Rectangle(2, 3) fails with “takes no arguments”. It is an easy typo to make and a hard one to spot.
• Making area an attribute instead of a method. The stub calls shape.area() with brackets. If you store self.area = 6, calling it raises “‘int’ object is not callable”.
• Returning a rounded or formatted value. Leave the formatting to the stub. Returning a string breaks its "%.2f" formatting.

Try it on sample data

Three queries, in the same format the stub reads:

3
rectangle 2 3
circle 1
circle 5

Output from my run of the solution with a copy of the stub’s loop:

6.00
3.14
78.54

Without the formatting, Circle(5).area() returns 78.53981633974483.