Use FastAPI for a Python API

Cost

Open source. MIT licence, free to run on your own servers.

How to do it

FastAPI first came out in December 2018, and version 0.156.8 shipped in October 2026. It has never reached version 1.0, so it fails the five-year stable test, and minor versions can break code. It passes the rest. It has about 114 million weekly downloads and an MIT licence. It was used by 14.8% of developers in the 2025 Stack Overflow survey, above Flask and Django. Record the exception as the first chapter asks. Choose something else when the app also needs pages, accounts and an admin, which Django brings. It checks input with Pydantic, as Use Zod or Pydantic to check every input says.

Examples

Using these examples: they show one way to follow this rule. Keep the behaviour and the check, not the syntax. Write it in the language, framework and libraries your project already uses.

Python example

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class Order(BaseModel):
    id: int
    total_cents: int

@app.get("/orders/{order_id}")
def get_order(order_id: int) -> Order:
    return Order(id=order_id, total_cents=4200)

JavaScript example

const PINNED_MINOR = /^fastapi>=0\.(\d+)(\.\d+)?,<0\.(\d+)$/;

export const isMinorPinned = requirement => {
  const match = PINNED_MINOR.exec(requirement.replace(/\s/g, ''));
  return Boolean(match) && Number(match[3]) === Number(match[1]) + 1;
};

How to check

requirements or pyproject pins FastAPI to one minor version, such as fastapi>=0.156,<0.157, and a decision record names the exception.

Sources

• FastAPI

Related rules

• Use Zod or Pydantic to check every input in Software choices