2.2.1 Starlette

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Book reference: Beyond the books. Original notes kept below; rewrite them in your own words.

2.2.1 Starlette

Starlette is a lightweight ASGI web framework for Python.

A useful way to think about it:

FastAPI
   ↓
Starlette
   ↓
ASGI
   ↓
Uvicorn

What does Starlette provide?

FastAPI is built on top of Starlette and uses Starlette for:

• Routing
• Requests and Responses
• Middleware
• WebSockets
• Background tasks
• Event loop integration
• ASGI support

For example, when you write:

from fastapi import FastAPI

app = FastAPI()

@app.get("/")
async def home():
    return {"message": "Hello"}

Behind the scenes:

• FastAPI extends Starlette.
• The route handling comes from Starlette.
• Uvicorn runs the application.

Starlette Without FastAPI

You can use Starlette directly:

from starlette.applications import Starlette
from starlette.responses import JSONResponse
from starlette.routing import Route

async def home(request):
    return JSONResponse({"message": "Hello"})

app = Starlette(routes=[
    Route("/", home)
])

This works, but you lose FastAPI features such as:

• Pydantic validation
• Automatic OpenAPI docs
• Dependency Injection (Depends)
• Request/response models

Why FastAPI Exists

Starlette gives you the web framework basics.

FastAPI adds:

Starlette
+ Pydantic validation
+ OpenAPI/Swagger docs
+ Dependency Injection
+ Better developer experience
= FastAPI

In Your FastAPI Projects

When you use:

from fastapi.middleware.cors import CORSMiddleware

or

from fastapi import Request

many of those components ultimately come from Starlette.

For backend interviews, a good one-line answer is:

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Starlette is the ASGI framework underneath FastAPI. It provides routing, middleware, request/response handling, WebSockets, and async support, while FastAPI builds on it by adding validation, dependency injection, and automatic API documentation.