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⚡ Frameworks

FastAPI Tutorial – Modern Python REST API Framework

FastAPI is the fastest growing Python web framework — high performance (comparable to Node.js and Go), automatic data validation with Pydantic, auto-generated OpenAPI (Swagger) documentation, and async support built-in. It is the top choice for building modern REST APIs in Python.

⏱️ 30 min read🎯 Advanced📅 Updated 2026

Installation and First App

Install FastAPI and uvicorn (ASGI server) and run your first API in minutes.

Python
# pip install fastapi uvicorn

# main.py
from fastapi import FastAPI

app = FastAPI()

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

@app.get("/items/{item_id}")
def get_item(item_id: int, q: str | None = None):
    return {"item_id": item_id, "query": q}

# Run:
# uvicorn main:app --reload
# Visit: http://127.0.0.1:8000/docs  ← Auto Swagger UI!
▶ Output
{"message": "Hello from FastAPI!"} {"item_id": 42, "query": null}

Pydantic Models – Request/Response Validation

Use Pydantic models to define and validate request bodies automatically.

Python
from fastapi import FastAPI
from pydantic import BaseModel, EmailStr
from typing import Optional

app = FastAPI()

class User(BaseModel):
    name: str
    email: str
    age: int
    bio: Optional[str] = None

@app.post("/users/", response_model=User)
def create_user(user: User):
    # user is already validated and typed
    return user

# FastAPI auto-validates: wrong type = 422 error
# POST /users/ with {"name":"Alice","email":"a@b.com","age":30}
▶ Output
{"name":"Alice","email":"a@b.com","age":30,"bio":null}

Dependency Injection

FastAPI's dependency injection system handles auth, DB connections, and shared logic.

Python
from fastapi import FastAPI, Depends, HTTPException
from fastapi.security import HTTPBearer

app = FastAPI()
security = HTTPBearer()

def get_current_user(token = Depends(security)):
    if token.credentials != "secret-token":
        raise HTTPException(status_code=401, detail="Invalid")
    return {"user": "alice"}

@app.get("/protected")
def protected_route(user = Depends(get_current_user)):
    return {"message": f"Welcome {user['user']}"}
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Async Support

FastAPI supports both sync and async endpoints natively.

Python
import asyncio
from fastapi import FastAPI
import httpx

app = FastAPI()

@app.get("/async-data")
async def get_data():
    async with httpx.AsyncClient() as client:
        response = await client.get("https://api.github.com")
    return response.json()
💡
Tip

Use async functions for I/O-bound operations (database queries, HTTP calls). FastAPI handles the event loop automatically.

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