FastAPI Internals

The Speaker

Marcelo Trylesinski

OSS Maintainer

What is FastAPI?

How do I run FastAPI?

uvicorn main:app or...

www.uvicorn.org

How do I run FastAPI?

hypercorn main:app or...

github.com/pgjones/hypercorn

How do I run FastAPI?

granian main:app

github.com/emmett-framework/granian

How the interactions happen?

Let's focus on...

But... How it really happens?

ASGI application

Scope = Dict[str, Any]
Receive = Callable[[], Awaitable[Dict[str, Any]]]
Send = Callable[[Dict[str, Any]], Awaitable[None]]

async def app(scope: Scope, receive: Receive, send: Send):
    ...

Simplest ASGI application

async def app(scope, receive, send):
    body = b"Hello world!"
    headers = [(b"content-type", b"text/plain"), (b"content-length", str(len(body)).encode())]

    await send({"type": "http.response.start", "status": 200, "headers": headers})
    await send({"type": "http.response.body", "body": body})

After the connection is established...

If the client sends a body...

Then... It's the application's turn!

Middleware Stack

Middleware Stack

from fastapi import FastAPI
from src.middleware import CustomMiddleware, AnotherCustomMiddleware

app = FastAPI()
app.add_middleware(CustomMiddleware)
app.add_middleware(AnotherCustomMiddleware)

Middleware Stack

Routing

Routing

Dependency Injection

Dependency Injection

Dependency Injection

async def dependency():
    return "Hello, World!"

def dependency():
    return "Hello, World!"

Input/Output Validation

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class Input(BaseModel):
    name: str
    age: int

class Output(Input):
    id: int

@app.post("/")
async def read_item(input: Input) -> Output:
    return await database_insert(input)

Input/Output Validation

WebSockets

WebSockets

Async vs Sync

When should I use...

Bonus: FastAPI Tips

https://github.com/Kludex/fastapi-tips

Try Logfire!

Thank You!

www.fastapiexpert.com

Marcelo Trylesinski
@marcelotryle
Kludex