Constructor parameters in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a class with default constructor parameters

What is the output of this Python code?

Python

class Car:
    def __init__(self, make, year=2020):
        self.make = make
        self.year = year

car1 = Car('Toyota')
car2 = Car('Honda', 2018)
print(car1.make, car1.year)
print(car2.make, car2.year)

A Error: missing argument for year

B

Toyota 2020
Honda 2018

C

Toyota 2020
Honda 2020

D

Toyota None
Honda 2018

💡 Hint

Look at the default value assigned to the year parameter in the constructor.

The constructor has a default value for year=2020. When car1 is created without specifying year, it uses 2020. car2 specifies 2018, so it uses that.

❓ Predict Output

intermediate

Output when using *args in constructor

What will this code print?

Python

class Numbers:
    def __init__(self, *args):
        self.nums = args

obj = Numbers(1, 2, 3)
print(obj.nums)

A Error: *args not allowed in constructor

B [1, 2, 3]

C {1, 2, 3}

D (1, 2, 3)

💡 Hint

Remember what *args collects in Python functions.

*args collects all extra positional arguments into a tuple. So self.nums is a tuple (1, 2, 3).

❓ Predict Output

advanced

Output of constructor with keyword-only parameters

What is the output of this code?

Python

class Person:
    def __init__(self, name, *, age):
        self.name = name
        self.age = age

p = Person('Alice', age=30)
print(p.name, p.age)

A Alice 30

B Alice None

C Error: missing required keyword-only argument 'age'

D Error: positional argument after keyword-only argument

💡 Hint

The * in the parameter list means age must be passed as a keyword argument.

The * forces age to be keyword-only. p is created with age=30 as keyword argument, so it works and prints 'Alice 30'.

❓ Predict Output

advanced

Output when constructor parameters are mutable default arguments

What will this code print?

Python

class Bag:
    def __init__(self, items=None):
        if items is None:
            items = []
        self.items = items

b1 = Bag()
b1.items.append('apple')
b2 = Bag()
print(b2.items)

A ['apple']

B ['apple', 'apple']

C []

D Error: mutable default argument not allowed

💡 Hint

Think about what happens when you use a list as a default parameter.

Using None as the default and creating a new list inside the constructor avoids sharing the list between instances. So b2.items is empty.

❓ Predict Output

expert

Output of constructor with parameter unpacking and match-case

What is the output of this code?

Python

class Shape:
    def __init__(self, **kwargs):
        self.type = kwargs.get('type', 'unknown')
        self.size = kwargs.get('size', 0)

s = Shape(type='circle', size=5)

match s.type:
    case 'circle':
        print(f'Circle with size {s.size}')
    case 'square':
        print(f'Square with size {s.size}')
    case _:
        print('Unknown shape')

A Circle with size 5

B Square with size 5

C Unknown shape

D Error: match-case not supported

💡 Hint

Look at how kwargs are unpacked and how match-case works with strings.

The constructor sets type='circle' and size=5. The match-case matches 'circle' and prints accordingly.