Folder structure patterns in FastAPI - Mini Project: Build & Apply

Folder Structure Patterns in FastAPI

📖 Scenario: You are building a simple FastAPI web application to manage books in a library. To keep your project clean and easy to maintain, you want to organize your code using a common folder structure pattern.

🎯 Goal: Build a basic FastAPI project with a clear folder structure that separates the main app, routers, and models.

📋 What You'll Learn

Create a main application file main.py inside the app folder

Create a routers folder inside app with a books.py router file

Create a models.py file inside app for data models

Include a simple route in books.py that returns a list of book titles

💡 Why This Matters

🌍 Real World

Organizing code in folders is essential for real FastAPI projects to keep code manageable and scalable.

💼 Career

Understanding folder structure patterns is a key skill for backend developers working with FastAPI or similar frameworks.

1

Create the initial folder structure and main app file

Create a folder called app. Inside app, create a file named main.py with the following code: import FastAPI from fastapi and create an instance called app.

FastAPI

# Create the app folder and main.py file
from fastapi import FastAPI

app = FastAPI()
# Your code here

Hint

Use from fastapi import FastAPI and then create app = FastAPI().

2

Add a routers folder and create books router

Inside the app folder, create a folder named routers. Inside routers, create a file called books.py. In books.py, import APIRouter from fastapi, create a router instance called router, and add a GET route /books that returns a list of book titles: ["Book A", "Book B"].

FastAPI

from fastapi import FastAPI

app = FastAPI()

# In routers/books.py
from fastapi import APIRouter

router = APIRouter()

# Your code here

Hint

Create router = APIRouter() and add a GET route @router.get("/books") that returns the list.

3

Create a models.py file for data models

Inside the app folder, create a file named models.py. In this file, import BaseModel from pydantic and create a class Book that inherits from BaseModel with two fields: title (string) and author (string).

FastAPI

# routers/books.py
from fastapi import APIRouter

router = APIRouter()

@router.get("/books")
async def get_books():
    return ["Book A", "Book B"]

# In models.py
from pydantic import BaseModel

# Your code here

Hint

Use class Book(BaseModel): with title: str and author: str fields.

4

Connect the books router in main.py

In main.py, import router from app.routers.books and include it in the app using app.include_router(router).

FastAPI

from fastapi import FastAPI
from app.routers.books import router

app = FastAPI()

# Your code here to include the router

Hint

Use app.include_router(router) to add the books routes to the main app.

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