Modifying object state in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this code modifying object attributes?

Look at the code below. What will be printed after running it?

Python

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

    def resize(self, new_length, new_width):
        self.length = new_length
        self.width = new_width

box = Box(5, 3)
box.resize(10, 6)
print(box.length, box.width)

A 10 3

B 5 3

C 10 6

D 5 6

💡 Hint

Think about what the resize method does to the object's attributes.

The resize method changes the length and width attributes of the box object to 10 and 6 respectively. So, printing them shows 10 6.

❓ Predict Output

intermediate

What will be the value of the counter after method calls?

Consider this class that modifies its internal counter. What is the final value printed?

Python

class Counter:
    def __init__(self):
        self.count = 0

    def increment(self):
        self.count += 1

    def reset(self):
        self.count = 0

c = Counter()
c.increment()
c.increment()
c.reset()
c.increment()
print(c.count)

A 0

B 1

C 3

D 2

💡 Hint

Follow the changes to count step by step.

The counter increments twice to 2, then resets to 0, then increments once more to 1. So the final value is 1.

❓ Predict Output

advanced

What is the output when modifying a mutable attribute inside a method?

What will this code print?

Python

class Basket:
    def __init__(self):
        self.items = []

    def add_item(self, item):
        self.items.append(item)

b = Basket()
b.add_item('apple')
b.add_item('banana')
print(b.items)

A ['apple', 'banana']

B ['apple']

C []

D ['banana']

💡 Hint

Think about how the list items changes when you append new elements.

The add_item method adds each item to the items list. After adding 'apple' and 'banana', the list contains both.

❓ Predict Output

advanced

What error occurs when modifying a non-existent attribute?

What happens when this code runs?

Python

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

car = Car('Sedan')
car.speed = 60
print(car.speed)
print(car.color)

A 60 followed by AttributeError

B 60 followed by 0

C AttributeError at car.speed assignment

D 60 followed by null

💡 Hint

Check which attributes exist and which do not.

Assigning car.speed = 60 creates the attribute speed. Printing it shows 60. But car.color was never set, so accessing it raises AttributeError.

🧠 Conceptual

expert

Which option correctly modifies the object's state to track history?

You want to keep track of all values assigned to an attribute value in an object. Which code correctly updates the history each time value changes?

A

class Tracker:
    def __init__(self):
        self.value = None
        self.history = []
    
    def update(self, new_value):
        self.value = new_value
        self.history = [new_value]

B

class Tracker:
    def __init__(self):
        self.value = None
        self.history = []
    
    def set_value(self, new_value):
        self.value = new_value
        self.history.append(new_value)

C

class Tracker:
    def __init__(self):
        self.value = None
        self.history = []
    
    def value(self, new_value):
        self.value = new_value
        self.history.append(new_value)

D

class Tracker:
    def __init__(self):
        self._value = None
        self.history = []
    
    @property
    def value(self):
        return self._value
    
    @value.setter
    def value(self, new_value):
        self._value = new_value
        self.history.append(new_value)

💡 Hint

Think about how to run code automatically when an attribute changes.

Option D uses a property setter to update _value and append to history every time value changes. This tracks all changes correctly.

Other options either don't use property setters or overwrite history incorrectly.