OOP Interview Drill and Checkpoint

60 min · Intermediate

Fix the mistakes

The slips that cost marks in this module. Spot each one, then check the fix.

Treat this page as a mock round. Predict each output before running it, fix the bug, then explain your answer out loud in two or three sentences.

Predict the output

Trace the code line by line. Ask where each name lives: instance, class, or a parent class. Then follow super() along the MRO.

Python

1class Base:2    log = []3
4    def run(self):5        self.log.append("base")6
7class Child(Base):8    def run(self):9        super().run()10        self.log.append("child")11
12Child().run()13Base().run()14print(Base.log)

Example 1 · Tracing attributes, properties and shadowing

Python

1class Account:2    bank = "MentorBank"3
4    def __init__(self, owner, balance):5        self.owner = owner6        self._balance = balance7
8    @property9    def balance(self):10        return self._balance11
12a = Account("Asha", 500)13b = Account("Ravi", 200)14a.bank = "OtherBank"15print(a.bank, b.bank, Account.bank)16print(a.balance + b.balance)17print(sorted(vars(a)))

Output

OtherBank MentorBank MentorBank
700
['_balance', 'bank', 'owner']

What this code does

Assigning a.bank created an instance attribute, so only a changed.
balance is a property and is not in vars(a); the stored value _balance is.

log is one list on Base, shared by every class below it. Nothing creates a new one, so each append adds to the same list.

Spot the bug

Most bug questions in OOP rounds use a shared default. The default is built once, when the function or class body runs.

Python

1class Basket:2    def __init__(self, items=[]):3        self.items = items4
5    def add(self, item):6        self.items.append(item)7
8a = Basket()9b = Basket()10a.add("pen")11print(b.items)

Example 2 · The fixed basket, with a safe equality

Python

1class Basket:2    def __init__(self, items=None):3        self.items = [] if items is None else list(items)4
5    def add(self, item):6        self.items.append(item)7
8    def __eq__(self, other):9        return isinstance(other, Basket) and self.items == other.items10
11    __hash__ = None12
13a = Basket()14b = Basket()15a.add("pen")16print(a.items, b.items)17print(a == b, a == Basket(["pen"]))

Output

['pen'] []
False True

What this code does

Each basket now gets its own list.
A mutable object with value equality sets __hash__ = None on purpose, so it cannot sit in a set by mistake.

Both baskets use the one default list. Use items=None and write self.items = [] if items is None else items.

Pick the design

Say the relationship as a sentence. "A Stack is a list" fails, because a stack must not allow inserting in the middle. "A Stack has a list" fits.

Python

1class Stack:2    def __init__(self):3        self._items = []4
5    def push(self, item):6        self._items.append(item)7
8    def pop(self):9        return self._items.pop()10
11    def __len__(self):12        return len(self._items)

Example 3 · A small design: payments with parts

Python

1from abc import ABC, abstractmethod2from dataclasses import dataclass3
4class Payment(ABC):5    @abstractmethod6    def pay(self, amount):7        ...8
9class CardPayment(Payment):10    def pay(self, amount):11        return f"card paid {amount}"12
13class UpiPayment(Payment):14    def pay(self, amount):15        return f"upi paid {amount}"16
17@dataclass18class Checkout:19    method: Payment20
21    def complete(self, amount):22        return self.method.pay(amount)23
24for m in (CardPayment(), UpiPayment()):25    print(Checkout(m).complete(250))

Output

card paid 250
upi paid 250

What this code does

Checkout has a payment method and never inherits from one.
Adding a wallet payment means one new class and no change to Checkout.

The caller sees only push, pop and len. The list is a private detail that can change later.

Module quiz

Questions from the whole module. Your first answer is your score.

Problems

Graded easy to hard, using everything from this module.

Try each one yourself first. Use the hint if you get stuck. Mark it as done when it works.

Exercise 1· Easy

Write Order with a class attribute count that grows with each new order. Create three orders and print Order.count.

Expected output

Exercise 2· Easy

Write Person(name) and Student(Person) with marks. Student.describe() must reuse the parent's describe() and return <name> scoring <marks>. Print it for Asha and 82.

Expected output

Asha scoring 82

Exercise 3· Easy

Write Customer(first, last) with a read-only property full_name returning <first> <last>. Print it for Asha Rao.

Expected output

Asha Rao

Exercise 4· Medium

Write add_item(item, items=None) that returns the list with the item added, using the safe default. Call it with a and then with b and print both results on separate lines.

Expected output

['a']
['b']

Exercise 5· Medium

Write a dataclass Team with name and a members list that defaults to a new empty list. Create two teams, add Asha to the first and print both teams' members, one per line.

Expected output

['Asha']
[]

Exercise 6· Medium

Write an abstract Notifier with send(text), then Email and Sms implementations returning email: <text> and sms: <text>. Write broadcast(notifiers, text) that prints each result. Call it with both for Order shipped.

Expected output

email: Order shipped
sms: Order shipped

Exercise 7· Hard

Write Temp(c) with a property f for Fahrenheit that has a getter and a setter. Setting f = 212 must change c. Print c after setting 212, then set c = 0 and print f.

Expected output

100.0
32.0

Exercise 8· Hard

Write Money(amount) with __add__, __eq__, __hash__, __lt__ and __repr__. Print sum([Money(5), Money(7)], Money(0)), then the sorted list [Money(9), Money(2), Money(5)], then the size of a set of Money(3) made twice.

Expected output

Money(12)
[Money(2), Money(5), Money(9)]
1

Interview questions

Questions interviewers ask on this topic.

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Next: Lists, Tuples and Slicing

1class Base:2 log = []3 4 def run(self):5 self.log.append("base")6 7class Child(Base):8 def run(self):9 super().run()10 self.log.append("child")11 12Child().run()13Base().run()14print(Base.log)

1class Account:2 bank = "MentorBank"3 4 def __init__(self, owner, balance):5 self.owner = owner6 self._balance = balance7 8 @property9 def balance(self):10 return self._balance11 12a = Account("Asha", 500)13b = Account("Ravi", 200)14a.bank = "OtherBank"15print(a.bank, b.bank, Account.bank)16print(a.balance + b.balance)17print(sorted(vars(a)))

1class Basket:2 def __init__(self, items=None):3 self.items = [] if items is None else list(items)4 5 def add(self, item):6 self.items.append(item)7 8 def __eq__(self, other):9 return isinstance(other, Basket) and self.items == other.items10 11 __hash__ = None12 13a = Basket()14b = Basket()15a.add("pen")16print(a.items, b.items)17print(a == b, a == Basket(["pen"]))

1class Stack:2 def __init__(self):3 self._items = []4 5 def push(self, item):6 self._items.append(item)7 8 def pop(self):9 return self._items.pop()10 11 def __len__(self):12 return len(self._items)

1from abc import ABC, abstractmethod2from dataclasses import dataclass3 4class Payment(ABC):5 @abstractmethod6 def pay(self, amount):7 ...8 9class CardPayment(Payment):10 def pay(self, amount):11 return f"card paid {amount}"12 13class UpiPayment(Payment):14 def pay(self, amount):15 return f"upi paid {amount}"16 17@dataclass18class Checkout:19 method: Payment20 21 def complete(self, amount):22 return self.method.pay(amount)23 24for m in (CardPayment(), UpiPayment()):25 print(Checkout(m).complete(250))

1class Base:2 log = []3 4 def run(self):5 self.log.append("base")6 7class Child(Base):8 def run(self):9 super().run()10 self.log.append("child")11 12Child().run()13Base().run()14print(Base.log)

1class Person:2 def __init__(self, name):3 self.name = name4 5 def describe(self):6 return self.name7 8class Student(Person):9 def __init__(self, name, marks):10 super().__init__(name)11 self.marks = marks12 13 def describe(self):14 return f"{super().describe()} scoring {self.marks}"15 16print(Student("Asha", 82).describe())

1class Temp:2 def __init__(self, c):3 self.c = c4 5 @property6 def f(self):7 return self.c * 9 / 5 + 328 9 @f.setter10 def f(self, value):11 self.c = (value - 32) * 5 / 912 13t = Temp(0)14t.f = 21215print(t.c)

1class Base:2 log = []3 4 def run(self):5 self.log.append("base")6 7class Child(Base):8 def run(self):9 super().run()10 self.log.append("child")11 12Child().run()13Base().run()14print(Base.log)

Predict the output

What this code does

Spot the bug

What this code does

Pick the design

What this code does

Exercise 1· Easy

Exercise 2· Easy

Exercise 3· Easy

Exercise 4· Medium

Exercise 5· Medium

Exercise 6· Medium

Exercise 7· Hard

Exercise 8· Hard

You finished the preview of OOP Questions

Predict the output

What this code does

Spot the bug

What this code does

Pick the design

What this code does

What does this code print?

What does this code print?

Which is the safest default for a list parameter?

What does this code print?

Why is class Stack(list) a poor design?

Which statement about @abstractmethod is true?

What does this code print?

Which sentence tells you to use composition?

What does this code print?

In class D(B, C), which class's super().run() is called by B.run?

A teammate writes two methods named area in one class with different parameters. What happens?

What does dataclass(frozen=True) give you besides blocking assignment?

Exercise 1· Easy

Exercise 2· Easy

Exercise 3· Easy

Exercise 4· Medium

Exercise 5· Medium

Exercise 6· Medium

Exercise 7· Hard

Exercise 8· Hard

Q1In 30 seconds: what are the four OOP pillars in Python?

Q2Explain self to a beginner.

Q3What are the three method types and when do you use each?

Q4What is the most common bug with class attributes?

Q5What is the most common bug with default arguments?

Q6How do you decide between inheritance and composition?

Q7What is the MRO and why does it matter?

Q8Why is super() better than naming the parent class?

Q9Does Python have private members?

Q10How do you validate an attribute?

Q11What happens when you instantiate an abstract class?

Q12Why does Python not have overloading?

Q13What is duck typing and what is its trade-off?

Q14Why must __eq__ and __hash__ go together?

Q15Dataclass or normal class?

Q16How would you make a custom class work with len, for and in?

Q17A class has 12 methods and five unrelated jobs. What do you do?

Q18How do you test a class that sends emails?

Q19What is a common design mistake about getters and setters?

Q20What would you review in a class design in 2 minutes?

Q21How do you explain a bug you made in a past OOP project?

You finished the preview of OOP Questions

What does this code print?

What does this code print?

Which is the safest default for a list parameter?

What does this code print?

Why is class Stack(list) a poor design?

Which statement about @abstractmethod is true?

What does this code print?

Which sentence tells you to use composition?

What does this code print?

In class D(B, C), which class's super().run() is called by B.run?

A teammate writes two methods named area in one class with different parameters. What happens?

What does dataclass(frozen=True) give you besides blocking assignment?

Q1In 30 seconds: what are the four OOP pillars in Python?

Q2Explain self to a beginner.

Q3What are the three method types and when do you use each?

Q4What is the most common bug with class attributes?

Q5What is the most common bug with default arguments?

Why is `class Stack(list)` a poor design?

Which statement about `@abstractmethod` is true?

In `class D(B, C)`, which class's `super().run()` is called by `B.run`?

A teammate writes two methods named `area` in one class with different parameters. What happens?

What does `dataclass(frozen=True)` give you besides blocking assignment?

Why is `class Stack(list)` a poor design?

Which statement about `@abstractmethod` is true?

In `class D(B, C)`, which class's `super().run()` is called by `B.run`?

A teammate writes two methods named `area` in one class with different parameters. What happens?

What does `dataclass(frozen=True)` give you besides blocking assignment?