String Representation of Objects

This question tests one dunder method, __str__. The logic is trivial. What fails people is a single wrong character in the output sentence.

What the question asks

Write two classes:

• Car, created with a maximum speed (an integer) and a speed unit (a string such as km/h or mph). When a Car is converted to a string, it should read Car with the maximum speed of <speed> <unit>.
• Boat, created with only a maximum speed. Boat speeds are always in knots, so a Boat converts to Boat with the maximum speed of <speed> knots.

The stub reads lines like car 151 km/h or boat 77, builds the object, and writes it with "%s" % vehicle, which calls your __str__.

Function to write

class Car:
    def __init__(self, max_speed, speed_unit): ...
    def __str__(self): ...

class Boat:
    def __init__(self, max_speed): ...
    def __str__(self): ...

Only the two classes are yours. Input reading and output are in HackerRank’s code.

Solution

class Car:
    def __init__(self, max_speed, speed_unit):
        self.max_speed = max_speed
        self.speed_unit = speed_unit

    def __str__(self):
        return f"Car with the maximum speed of {self.max_speed} {self.speed_unit}"


class Boat:
    def __init__(self, max_speed):
        self.max_speed = max_speed

    def __str__(self):
        return f"Boat with the maximum speed of {self.max_speed} knots"

How it works

• The constructors just store what they are given.
• print(obj), str(obj), f-strings and "%s" % obj all call obj.__str__() to get the text. Without it, Python prints something like <__main__.Car object at 0x10...>.
• The f-string drops the stored values into the fixed sentence. The speed is an int, and the f-string converts it for you.

Every public solution I checked uses exactly these two sentences, with no full stop at the end.

Mistakes that fail the hidden tests

• Any change in the sentence. “Maximum” with a capital M, “max speed”, a full stop at the end, “knot” instead of “knots”, or two spaces somewhere. The checker compares the text exactly. Copy the sentence carefully from the question on test day.
• Printing inside __str__. __str__ must return the string. If it prints and returns nothing, Python raises “__str__ returned non-string (type NoneType)”.
• Writing the unit into the Car string. The unit changes from test to test (km/h, mph and so on). Only the Boat has a fixed unit.
• Using __new__ to return a string. Some public answers do this. It passes because the stub only prints the result, but the “object” is then just a string and not a Car. Use __str__, which is what the question is testing.

Defining __repr__ instead of __str__ also passes, because Python falls back to __repr__ when __str__ is missing. __str__ is the right method for human-readable text, so use that.

Try it on sample data

Input in the stub’s format:

2
car 151 km/h
boat 77

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

Car with the maximum speed of 151 km/h
Boat with the maximum speed of 77 knots