FastAPI is a modern Python framework for building REST APIs.

It is fast, easy to use, and provides automatic validation and API documentation.

This guide covers 5 essential concepts you need to get started.

Final Picture

The 5 concepts work together:

Map
                FastAPI
                   │
        ┌──────────┼──────────┐
        ↓          ↓          ↓
      HTTP      Pydantic    Router
        │          │          │
        └──────────┼──────────┘
                   ↓
              Business Logic
                   ↓
               Database
                   ↓
                Response
                   ↓
                Testing
                   ↓
               Deployment

1. First Steps

What is it?

A FastAPI app starts with a FastAPI instance and defines API endpoints using decorators.

from fastapi import FastAPI

app = FastAPI()


@app.get("/")
def root():
    return {"message": "Hello FastAPI"}

When to use it?

Whenever you need to create an API endpoint.

For example:

GET /products
GET /products/1
POST /products

2. Pydantic Models

What is it?

Pydantic models define the structure of your API data and automatically validate it.

from pydantic import BaseModel


class ProductCreate(BaseModel):
    name: str
    price: float

Now FastAPI knows what the request body should look like.

{
  "name": "Coconut",
  "price": 100000
}

When to use it?

Use Pydantic models when receiving or returning structured data.

@app.post("/products")
def create_product(product: ProductCreate):
    return product

If the data is invalid, FastAPI automatically returns a validation error.

Response Models

You can also define the response:

class ProductResponse(BaseModel):
    id: int
    name: str
    price: float

@app.get(
    "/products/{product_id}",
    response_model=ProductResponse
)
def get_product(product_id: int):
    return {
        "id": product_id,
        "name": "Coconut",
        "price": 100000
    }

3. Database

What is it?

FastAPI handles the API layer, while a database stores persistent data.

A typical application looks like:

Next.js
   ↓
FastAPI
   ↓
PostgreSQL

FastAPI receives the request, performs business logic, and communicates with the database.

When to use it?

Use a database when data needs to persist after the application restarts.

For example:

Products
Users
Orders
Categories
Payments

A common stack is:

FastAPI
+
SQLAlchemy
+
PostgreSQL
+
Alembic

Example flow:

POST /products
      ↓
FastAPI
      ↓
Validate with Pydantic
      ↓
Service
      ↓
SQLAlchemy
      ↓
PostgreSQL

Pydantic models and database models have different purposes:

Pydantic Model
→ API data

SQLAlchemy Model
→ Database data

4. HTTP & Authentication

HTTP

HTTP defines how the client and server communicate.

Common methods:

GET     → Read
POST    → Create
PUT     → Replace
PATCH   → Update
DELETE  → Delete

Common status codes:

200 → OK
201 → Created
204 → No Content

400 → Bad Request
401 → Not Authenticated
403 → Forbidden
404 → Not Found
422 → Validation Error

500 → Server Error

Authentication

Authentication answers:

Who are you?

For example, a user logs in:

Next.js
   ↓
POST /login
   ↓
FastAPI
   ↓
Verify credentials
   ↓
Create token
   ↓
Browser

A common approach is:

JWT
+
HttpOnly Cookie

Authentication is especially important for protected endpoints:

/admin
/users
/orders
/profile

5. Testing & Deployment

Testing

Testing verifies that your API works as expected.

FastAPI works well with:

pytest
+
TestClient

Example:

from fastapi.testclient import TestClient

from main import app


client = TestClient(app)


def test_get_products():
    response = client.get("/products")

    assert response.status_code == 200

Run tests:

pytest

Important things to test:

Authentication
Validation
CRUD
Business Logic
Error Handling

Deployment

A common production setup is:

Internet
   ↓
Reverse Proxy
   ↓
FastAPI
   ↓
PostgreSQL

Docker makes it easier to package and run the application:

Docker
├── FastAPI
└── PostgreSQL

For example:

docker compose up --build

This builds the containers and starts the application.