Folder structure patterns in FastAPI - Cheat Sheet & Quick Revision

Recall & Review

beginner

Why is organizing your FastAPI project into folders important?

It helps keep code clean, easy to find, and simple to maintain as the project grows, just like organizing files in labeled folders on your computer.

beginner

What is the purpose of the app/main.py file in a FastAPI project?

It usually contains the main FastAPI app instance and the code to start the server, acting like the front door to your application.

beginner

What kind of code do you put inside the app/api/ folder?

You put your route definitions and API endpoint logic here, similar to how a restaurant menu lists all the dishes you can order.

intermediate

Why separate models, schemas, and services into different folders?

Separating them keeps your code organized by role: models handle data structure, schemas handle data validation, and services handle business logic, like having different drawers for tools, papers, and gadgets.

intermediate

What is a common folder structure pattern for a medium-sized FastAPI project?

A typical pattern is: app/main.py, app/api/ for routes, app/models/ for database models, app/schemas/ for data validation, app/services/ for business logic, and app/core/ for config and utilities.

Which folder usually contains your API route definitions in a FastAPI project?

A app/models/

B app/core/

C app/api/

D app/tests/

The app/api/ folder is where you define your API routes and endpoints.

Where should you put your Pydantic schemas for data validation?

A app/schemas/

B app/services/

C app/api/

D app/models/

Pydantic schemas for validating and serializing data are typically placed in app/schemas/.

What is the role of the app/core/ folder?

A To keep database models

B To hold API route handlers

C To write unit tests

D To store configuration and utility functions

app/core/ usually contains configuration settings and helper utilities.

Why is it good to separate business logic into a services/ folder?

A To keep logic separate from routes and models for easier maintenance

B To store HTML templates

C To hold database migration files

D To write test cases

Separating business logic into services/ helps keep code clean and easier to update.

Which file usually starts the FastAPI application?

A app/services/auth.py

B app/main.py

C app/api/routes.py

D app/models/user.py

app/main.py typically creates the FastAPI app instance and runs the server.

Describe a simple folder structure for a FastAPI project and explain what each folder is for.

Think about separating code by its role to keep things organized.

Why is it helpful to separate your FastAPI code into folders like models, schemas, and services?

Consider how organizing your desk helps you find things faster.

Practice

1. Which folder is best suited to keep your API endpoint definitions in a FastAPI project?

easy

A. routes

B. models

C. services

D. database

Solution

1. Step 1: Understand folder roles in FastAPI

   The routes folder is typically used to store API endpoint definitions, organizing the paths and HTTP methods.

2. Step 2: Match folder purpose to content

   models is for data models, services for business logic, and database for database connection code.

3. Final Answer:

   routes -> Option A

4. Quick Check:

   API endpoints go in routes [OK]

Hint: API paths belong in routes folder [OK]

Common Mistakes:

• Putting endpoints in models folder
• Mixing database code with routes
• Storing business logic in routes

2. Which of the following is the correct way to organize a FastAPI project folder for database models?

easy

A. Store models in the root project folder

B. Place all models inside a folder named services

C. Keep models mixed with route files

D. Put database models inside a folder named models

Solution

1. Step 1: Identify the folder for data models

   Database models represent data structure and belong in a dedicated models folder for clarity and separation.

2. Step 2: Evaluate other options

   services is for business logic, mixing models with routes or root folder reduces clarity and maintainability.

3. Final Answer:

   Put database models inside a folder named models -> Option D

4. Quick Check:

   Models folder holds data models [OK]

Hint: Models go in models folder, not services or root [OK]

Common Mistakes:

• Confusing services with models
• Mixing models and routes
• Scattering models in root folder

3. Given this FastAPI folder structure:

app/
  routes/
    user.py
  models/
    user.py
  services/
    user_service.py

Which file should contain the function to fetch user data from the database?

medium

A. routes/user.py

B. models/user.py

C. services/user_service.py

D. app/__init__.py

Solution

1. Step 1: Understand folder responsibilities

   routes handles API endpoints, models defines data structures, and services contains business logic like fetching data.

2. Step 2: Match function purpose to folder

   Fetching user data is business logic, so it belongs in services/user_service.py.

3. Final Answer:

   services/user_service.py -> Option C

4. Quick Check:

   Data fetching logic goes in services [OK]

Hint: Business logic like fetching data belongs in services [OK]

Common Mistakes:

• Putting fetching logic in routes
• Adding functions to models instead of services
• Using __init__.py for logic

4. You see this folder structure in a FastAPI project:

app/
  routes.py
  models.py
  services.py
  database.py

What is a likely problem with this structure as the project grows?

medium

A. Too many files in root, making it hard to maintain

B. Using .py files instead of folders is invalid in FastAPI

C. Missing a templates folder for HTML rendering

D. Database code should be inside routes.py

Solution

1. Step 1: Analyze flat file structure

   Having all files directly under app/ is okay for small projects but becomes cluttered and hard to maintain as code grows.

2. Step 2: Understand folder benefits

   Using folders like routes/, models/, and services/ helps organize code by purpose and improves maintainability.

3. Final Answer:

   Too many files in root, making it hard to maintain -> Option A

4. Quick Check:

   Flat structure hurts maintainability [OK]

Hint: Avoid many files in root; use folders for organization [OK]

Common Mistakes:

• Thinking .py files can't be used
• Confusing database code location
• Assuming templates folder is mandatory

5. You want to organize a FastAPI project that has multiple features like users, products, and orders. Which folder structure pattern best supports scalability and maintainability?

hard

A. Mix routes and models in the same folder to reduce folder count

B. Create separate folders for each feature with their own routes, models, and services inside

C. Keep all code files in the root folder for easy access

D. Put all routes in one routes folder, all models in one models folder, and all services in one services folder

Solution

1. Step 1: Understand feature-based folder structure

   Organizing by feature means each feature (users, products, orders) has its own folder containing routes, models, and services, improving modularity.

2. Step 2: Compare with other patterns

   Grouping all routes or models together can become large and hard to maintain. Keeping all files in root or mixing unrelated code reduces clarity.

3. Final Answer:

   Create separate folders for each feature with their own routes, models, and services inside -> Option B

4. Quick Check:

   Feature-based folders improve scalability [OK]

Hint: Use feature folders with routes, models, services inside [OK]

Common Mistakes:

• Using flat folders for all code
• Mixing unrelated code in one folder
• Ignoring modular design for growth