Building an API with FastAPI

What is FastAPI?

If you come from the Node.js world you already know the pattern: a lightweight framework that maps HTTP verbs and URL paths to handler functions, handles request parsing, and returns JSON responses. FastAPI does exactly that for Python — but with a few superpowers baked in from the start.

FastAPI is built on two libraries you will get to know well:

Starlette — an ASGI web toolkit (the async equivalent of Node’s http module).
Pydantic — a data-validation library powered by Python type hints (think Zod + class-transformer rolled into one).

The result is a framework that generates OpenAPI docs automatically, validates request bodies and query parameters with zero boilerplate, and runs fully async by default.

FastAPI vs Express vs NestJS

// Express — manual everything
const express = require('express');
const app = express();
app.use(express.json());

app.get('/items/:id', (req, res) => {
  const id = Number(req.params.id); // manual cast
  if (isNaN(id)) return res.status(422).json({ error: 'bad id' });
  res.json({ id, name: 'Widget' });
});

app.listen(3000);

// NestJS — decorators + DI, similar concept
// @Get(':id') findOne(@Param('id') id: string) { ... }

# FastAPI — types do the work
from fastapi import FastAPI

app = FastAPI()

@app.get("/items/{item_id}")
async def get_item(item_id: int):   # int cast + 422 on failure — automatic
    return {"id": item_id, "name": "Widget"}

# Run with:
#   uvicorn main:app --reload
# Docs at: http://localhost:8000/docs

Key differences at a glance:

Feature	Express	NestJS	FastAPI
Type safety	Manual / TS compiler	TS + class-validator	Python type hints
Validation	Manual / Joi / Zod	class-validator	Pydantic (automatic)
Async model	Callbacks / Promises	Promises / Observables	`async/await` (ASGI)
OpenAPI docs	swagger-jsdoc	@nestjs/swagger	Built-in, automatic
DI container	None built-in	Yes (IoC)	`Depends()` (lightweight)
Performance	Fast	Moderate	Very fast (Starlette)

What you will build

Throughout this module you will construct a small Items API — the same kind of CRUD service you have probably built a dozen times in Node. Along the way you will cover:

Project layout and startup
Routing, path params, and query params
Pydantic models for request / response schemas
Automatic validation and constraints
Response models and status codes
Dependency injection with Depends
Middleware and CORS
Error handling with HTTPException
Database integration with SQLAlchemy
Testing with TestClient + pytest

Run this locally — needs FastAPI + a server. Install the stack: pip install fastapi uvicorn[standard] pydantic sqlalchemy pytest httpx