Introduction to FastAPI

FastAPI is a modern, fast (high-performance) Python web framework for building APIs. It's designed to be easy to use, fast to code, and ready for production. FastAPI is based on Starlette for the web parts and Pydantic for the data parts, making it a powerful tool for building RESTful APIs.

Let's dive into the key features and concepts of FastAPI:

Setting Up FastAPI

First, install FastAPI and its dependencies:

pip install fastapi[all]

Now, let's create a simple "Hello, World!" API:

from fastapi import FastAPI

app = FastAPI()

@app.get("/")
async def root():
    return {"message": "Hello, World!"}

To run the server:

uvicorn main:app --reload

Visit http://localhost:8000 in your browser, and you'll see the JSON response.

Request Handling and Path Parameters

FastAPI makes it easy to handle different HTTP methods and path parameters:

from fastapi import FastAPI

app = FastAPI()

@app.get("/items/{item_id}")
async def read_item(item_id: int):
    return {"item_id": item_id}

In this example, item_id is a path parameter that FastAPI will automatically convert to an integer.

Query Parameters

You can easily define query parameters:

from fastapi import FastAPI

app = FastAPI()

@app.get("/items/")
async def read_items(skip: int = 0, limit: int = 10):
    return {"skip": skip, "limit": limit}

Now, you can call /items/?skip=20&limit=30 to customize the query.

Request Body

FastAPI uses Pydantic models to define request bodies:

from fastapi import FastAPI
from pydantic import BaseModel

class Item(BaseModel):
    name: str
    description: str = None
    price: float
    tax: float = None

app = FastAPI()

@app.post("/items/")
async def create_item(item: Item):
    return item

This setup automatically validates the incoming JSON data against the Item model.

Response Models

You can use Pydantic models to define your API's response structure:

from fastapi import FastAPI
from pydantic import BaseModel

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

app = FastAPI()

@app.get("/items/{item_id}", response_model=Item)
async def read_item(item_id: int):
    return {"name": "Foo", "price": 42.0}

This ensures that your API always returns the expected data structure.

Dependency Injection

FastAPI's dependency injection system is powerful and flexible:

from fastapi import FastAPI, Depends, HTTPException

app = FastAPI()

async def verify_token(token: str):
    if token != "secret-token":
        raise HTTPException(status_code=400, detail="Invalid token")
    return token

@app.get("/items/")
async def read_items(token: str = Depends(verify_token)):
    return {"token": token}

This example demonstrates how to use dependency injection for authentication.

Async Support

FastAPI is built on top of Starlette, which means it has excellent support for asynchronous programming:

import asyncio
from fastapi import FastAPI

app = FastAPI()

@app.get("/")
async def root():
    await asyncio.sleep(1)

# Simulate an I/O-bound operation
    return {"message": "Hello, World!"}

This allows your API to handle many concurrent requests efficiently.

OpenAPI (Swagger) Documentation

FastAPI automatically generates OpenAPI (formerly Swagger) documentation for your API. Visit /docs to see the interactive API documentation and /redoc for an alternative view.

Error Handling

FastAPI provides a clean way to handle errors:

from fastapi import FastAPI, HTTPException

app = FastAPI()

@app.get("/items/{item_id}")
async def read_item(item_id: int):
    if item_id == 42:
        raise HTTPException(status_code=404, detail="Item not found")
    return {"item_id": item_id}

This example shows how to raise an HTTP exception with a custom status code and message.

Advanced Features

FastAPI offers many advanced features, including:

1. Background tasks
2. WebSockets
3. CORS (Cross-Origin Resource Sharing)
4. Middleware support
5. File uploads
6. Security and authentication utilities

Here's a quick example of setting up CORS:

from fastapi import FastAPI
from fastapi.middleware.cors import CORSMiddleware

app = FastAPI()

app.add_middleware(
    CORSMiddleware,
    allow_origins=["*"],
    allow_credentials=True,
    allow_methods=["*"],
    allow_headers=["*"],
)

This setup allows all origins, which is suitable for development but should be restricted in production.