fastapi/fastapi

FastAPI – High‑performance Python web framework for APIs

What it is – FastAPI is a modern, production‑ready web framework for building HTTP APIs with Python. It leverages standard Python type hints, Starlette for the async web core, and Pydantic for data validation/serialization.

Why it matters – The framework is engineered for speed (comparable to Node.js or Go) while keeping the developer experience simple: you write ordinary Python functions, get automatic request validation, serialization, and interactive OpenAPI docs (Swagger UI & ReDoc) for free.

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Core features (as listed in the README)

• Performance – Built on Starlette and Pydantic; one of the fastest Python API frameworks.
• Fast to code – Typical development speed gains of 200‑300 %.
• Fewer bugs – Roughly 40 % reduction in developer‑induced errors thanks to automatic validation.
• Intelligent editor support – Full type‑aware autocompletion and linting.
• Standards‑based – Generates OpenAPI (Swagger) and JSON‑Schema specifications automatically.
• Automatic docs – Interactive Swagger UI and ReDoc interfaces are served out‑of‑the‑box.
• Dependency injection – Simple, type‑based DI system for clean architecture.
• Security helpers – Built‑in OAuth2, JWT, HTTP Basic, etc.
• Extensible – Supports WebSockets, CORS, cookie sessions, GraphQL (via Strawberry), and more via Starlette extensions.

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Typical use cases

• Micro‑services and internal APIs.
• Public RESTful services for machine‑learning models or data pipelines.
• Backend for web or mobile applications where rapid iteration is needed.
• Prototyping and production deployments that require automatic OpenAPI docs.

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Getting started (from the README)

# Install with uv (or pip) – the "standard" extra pulls in recommended dependencies
uv add "fastapi[standard]"

Create main.py:

from fastapi import FastAPI

app = FastAPI()

@app.get('/')
def read_root():
    return {"Hello": "World"}

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

Run the development server:

uv run fastapi dev   # auto‑reload, serves at http://127.0.0.1:8000

Visit http://127.0.0.1:8000/docs (Swagger UI) or .../redoc for the alternative UI.

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Extending the example (PUT with a body)

from pydantic import BaseModel

class Item(BaseModel):
    name: str
    price: float
    is_offer: bool | None = None

@app.put('/items/{item_id}')
def update_item(item_id: int, item: Item):
    return {"item_name": item.name, "item_id": item_id}

The docs update automatically to show the new request body schema.

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Deployment options

• FastAPI Cloud – one‑click deployment via uv run fastapi deploy (provided by the same team).
• Any ASGI‑compatible host (Uvicorn, Hypercorn, Gunicorn with Uvicorn workers, Docker, serverless platforms, etc.).

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Who uses it

The README cites production use at Microsoft, Uber (Ludwig), Netflix (Dispatch), Cisco, and many other companies.

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TL;DR

FastAPI lets you write standard Python functions, annotates them with type hints, and instantly gets:

• high‑performance async handling,
• automatic request validation & response serialization,
• live OpenAPI documentation,
• strong editor support. All with a tiny learning curve, making it a go‑to choice for modern Python APIs.