Why User, Group, Expense classes in LLD? - Purpose & Use Cases

The Big Idea

What if you could stop the headache of messy shared bills with just a few smart classes?

The Scenario

Imagine you are trying to track who owes what in a group of friends manually using just notes or spreadsheets.

You write down each person's name, the groups they belong to, and every expense they make. It quickly becomes a mess.

The Problem

Manually managing users, groups, and expenses is slow and confusing.

It's easy to forget who paid for what or how to split costs fairly.

Errors happen often, and updating information takes too much time.

The Solution

Using User, Group, and Expense classes organizes this data clearly.

Each user is an object, groups hold users, and expenses link to both.

This structure makes tracking, updating, and calculating shares simple and reliable.

Before vs After

✗ Before

users = ['Alice', 'Bob']
groups = {'Trip': ['Alice', 'Bob']}
expenses = [('Alice', 'Trip', 100)]

✓ After

class User:
    pass
class Group:
    pass
class Expense:
    pass

What It Enables

It enables building clear, scalable systems to manage shared expenses without confusion or errors.

Real Life Example

Apps like Splitwise use these classes to help friends split bills easily and fairly.

Key Takeaways

Manual tracking is error-prone and slow.

Classes organize users, groups, and expenses logically.

This design makes managing shared costs easy and scalable.

Practice

1. Which class is primarily responsible for storing information about individual people in a shared expense system?

easy

A. Payment

B. User

C. Expense

D. Group

Solution

1. Step 1: Understand the role of each class

   User class represents individual people, Group holds multiple users, Expense tracks costs.

2. Step 2: Identify which class stores individual info

   User class stores personal details like name and ID for each person.

3. Final Answer:

   User -> Option B

4. Quick Check:

   User = Individual person [OK]

Hint: User class = individual person info [OK]

Common Mistakes:

• Confusing Group with User
• Thinking Expense stores user details
• Assuming Payment is a class here

2. Which of the following is the correct way to define a Group class that holds multiple User objects in Python?

easy

A. class Group:
def __init__(self):
self.users = []

B. class Group:
def __init__(self):
self.user = {}

C. class Group:
def __init__(self):
self.expenses = []

D. class Group:
def __init__(self):
self.members = None

Solution

1. Step 1: Identify correct attribute for multiple users

   A list is suitable to hold multiple User objects, so self.users = [] is correct.

2. Step 2: Check other options for correctness

   class Group: def __init__(self): self.user = {} uses a dict named user which is not typical for holding users; class Group: def __init__(self): self.expenses = [] uses expenses which belongs to Expense class; class Group: def __init__(self): self.members = None sets members to None which is not a collection.

3. Final Answer:

   class Group: def __init__(self): self.users = [] -> Option A

4. Quick Check:

   Group holds list of users = self.users = [] [OK]

Hint: Group holds list of users with self.users = [] [OK]

Common Mistakes:

• Using dict instead of list for users
• Confusing expenses with users
• Initializing members as None instead of a list

3. Given the following code snippet, what will be the output?

class Expense:
    def __init__(self, amount, paid_by, split_between):
        self.amount = amount
        self.paid_by = paid_by
        self.split_between = split_between

    def split_amount(self):
        return self.amount / len(self.split_between)

expense = Expense(120, 'Alice', ['Alice', 'Bob', 'Charlie'])
print(expense.split_amount())

medium

A. Error

B. 60

C. 120

D. 40.0

Solution

1. Step 1: Understand the split_amount method

   It divides total amount by number of people in split_between list.

2. Step 2: Calculate the split

   Amount = 120, split_between has 3 people, so 120 / 3 = 40.0.

3. Final Answer:

   40.0 -> Option D

4. Quick Check:

   120 divided by 3 = 40.0 [OK]

Hint: Divide amount by count of split_between list [OK]

Common Mistakes:

• Forgetting to divide by number of people
• Dividing by 2 instead of 3
• Assuming paid_by affects split amount

4. Identify the bug in this Expense class method that calculates each user's share:

class Expense:
    def __init__(self, amount, paid_by, split_between):
        self.amount = amount
        self.paid_by = paid_by
        self.split_between = split_between

    def split_amount(self):
        return self.amount // len(self.split_between)

medium

A. Using integer division (//) may lose cents in split

B. split_between should be a dictionary, not a list

C. paid_by should be a list, not a single user

D. amount should be a string, not a number

Solution

1. Step 1: Analyze the division operator used

   The method uses integer division (//) which truncates decimals.

2. Step 2: Understand impact on money split

   Using // can lose fractional cents, causing inaccurate splits.

3. Final Answer:

   Using integer division (//) may lose cents in split -> Option A

4. Quick Check:

   Integer division truncates decimals, causing loss [OK]

Hint: Use float division (/) for accurate money splits [OK]

Common Mistakes:

• Ignoring decimal loss from integer division
• Confusing data types for paid_by or split_between
• Thinking amount should be string

5. You want to design a system where multiple users in a group can add expenses, and the system automatically calculates how much each user owes or is owed. Which design approach best supports scalability and clear responsibility?

hard

A. Make User class handle all expense calculations and group management to centralize logic

B. Use only a single Expense class that stores all users and groups inside it to simplify design

C. Create User, Group, and Expense classes where Group manages users and expenses; Expense tracks amount and split; User tracks individual balances updated by Group

D. Store all data in a flat file and calculate splits manually each time without classes

Solution

1. Step 1: Identify responsibilities for each class

   User holds personal info and balances, Group manages users and expenses, Expense tracks costs and splits.

2. Step 2: Evaluate design for scalability and clarity

   Create User, Group, and Expense classes where Group manages users and expenses; Expense tracks amount and split; User tracks individual balances updated by Group cleanly separates concerns, making it easier to maintain and scale.

3. Final Answer:

   Create User, Group, and Expense classes where Group manages users and expenses; Expense tracks amount and split; User tracks individual balances updated by Group -> Option C

4. Quick Check:

   Clear class roles = scalable design [OK]

Hint: Separate concerns: User, Group, Expense each handle distinct roles [OK]

Common Mistakes:

• Putting all logic in one class
• Ignoring separation of concerns
• Using flat files for complex data