FastAPI Internals

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The Speaker

Marcelo Trylesinski

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OSS Maintainer

### Uvicorn 

### Starlette 

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What is FastAPI?

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How do I run FastAPI?

uvicorn main:app or...

www.uvicorn.org

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How do I run FastAPI?

hypercorn main:app or...

github.com/pgjones/hypercorn

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How do I run FastAPI?

granian main:app

github.com/emmett-framework/granian

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How the interactions happen?

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Let's focus on...

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But... How it really happens?

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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):
    ...

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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})

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After the connection is established...

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If the client sends a body...

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

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Middleware Stack

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Middleware Stack

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

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

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Middleware Stack

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Routing

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Routing

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Dependency Injection

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Dependency Injection

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Dependency Injection

### Async

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

### Sync

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! [marcelotryle.com](https://www.marcelotryle.com) Marcelo Trylesinski @marcelotryle Kludex