FastAPI Complete Guide: From Beginner to Building Your First API

Of course! API stands for Application Programming Interface. That sounds complicated, but the idea is simple.

FastAPI is a modern Python web framework for building APIs. The "Fast" in its name isn't just for show! Here's why it's so popular:

Not at all! If you have a basic understanding of Python syntax (variables, functions, dictionaries), you're ready to start.

FastAPI leverages modern Python features, so it's also a great way to learn and practice them in a real-world project!

This is a brilliant question that highlights a key difference between the ecosystems! Your analogy is spot on:

• `Express.js` (the web framework) is like `FastAPI`.
• `Node.js` (the runtime environment) is like `Python` itself.

So why the extra piece, `Uvicorn`? In Python, the server and the framework are decoupled. They talk to each other using a standard called ASGI (Asynchronous Server Gateway Interface).

It's not magic! The speed comes from smart design and modern Python features.

You use "decorators" like `@app.get()` to define the URL path and the HTTP method (`GET`, `POST`, `PUT`, `DELETE`).

To capture parts of the URL, you use curly braces `{}`. The value is then passed as an argument to your function.

@app.get("/items/{item_id}")
def read_item(item_id: int):
    return {"item_id": item_id}

Try adding this to your `main.py` and then visit http://127.0.0.1:8000/items/5. You'll see `{"item_id":5}`. Notice we used `item_id: int`. FastAPI uses this type hint to validate that the ID is an integer! If you visit `/items/foo`, you'll get a nice JSON error message.

Parameters that are not part of the path are automatically interpreted as "query" parameters. Just declare them as function arguments.

# /users?skip=0&limit=10
@app.get("/users")
def read_users(skip: int = 0, limit: int = 10):
    return {"skip": skip, "limit": limit}

Here, `skip` and `limit` are optional query parameters with default values. Visit http://127.0.0.1:8000/users?skip=5&limit=20 to see it work.

This is where FastAPI truly shines, with the help of a library called Pydantic. You define the "shape" of the data you expect using a simple Python class.

from pydantic import BaseModel

# 1. Define your data model as a class
class Item(BaseModel):
    name: str
    description: str | None = None # Python 3.10+ optional syntax
    price: float
    tax: float | None = None

# 2. Use it in your path operation
@app.post("/items/")
def create_item(item: Item):
    return item

All that with just a few lines of code. This is what makes developing with FastAPI so fast and robust.

Think of FastAPI as the spiritual successor to Flask, built for the modern, asynchronous world.

Django is a fantastic, mature framework, but it was designed for a different purpose: building traditional, full-stack, mostly server-rendered web applications. FastAPI is a specialist, and its specialty is building APIs.

You can even get the best of both worlds! Many teams use FastAPI for their high-performance API endpoints while still using the Django ORM and scripts for other tasks.

FastAPI isn't a silver bullet. The best developers have a full toolbox.

• Stick with Django for: Rapid development of traditional monolithic applications, especially when you heavily rely on its admin panel, templating system, and all-in-one ecosystem. If your app is 90% server-rendered pages and 10% API, Django is a great choice.
• Stick with Flask for: Very simple microservices or when you are a purist who wants to build everything from the ground up, choosing every single component yourself.
• Choose FastAPI for: Building any kind of API, from small microservices to large enterprise applications, where performance, type safety, and clear documentation are priorities.

A web server is like a restaurant waiter that handles requests from customers (browsers) and serves them the right food (data).

Modern web apps are like a restaurant with two kitchens! One kitchen serves the customers directly (frontend), and another kitchen prepares the ingredients (backend API).

// React component making API call to FastAPI
import React, { useState, useEffect } from 'react';

function UserList() {
  const [users, setUsers] = useState([]);

  useEffect(() => {
    // Frontend calls your FastAPI backend
    fetch('http://localhost:8000/api/users')
      .then(response => response.json())
      .then(data => setUsers(data));
  }, []);

  return (
    

      
Users
      {users.map(user => (
        
{user.name}
      ))}
    
  );
}

# FastAPI endpoint that React calls
from fastapi import FastAPI
from fastapi.middleware.cors import CORSMiddleware

app = FastAPI()

# Allow frontend to make requests
app.add_middleware(
    CORSMiddleware,
    allow_origins=["http://localhost:3000"],  # React dev server
    allow_credentials=True,
    allow_methods=["*"],
    allow_headers=["*"],
)

@app.get("/api/users")
async def get_users():
    # Your business logic here
    return [
        {"id": 1, "name": "John Doe"},
        {"id": 2, "name": "Jane Smith"}
    ]

Development vs Production is like cooking at home vs running a restaurant!

There are several ways to deploy, depending on your needs:

# Serving React build files from FastAPI
from fastapi import FastAPI
from fastapi.staticfiles import StaticFiles
from fastapi.responses import FileResponse

app = FastAPI()

# Mount the React build folder
app.mount("/static", StaticFiles(directory="react-app/build/static"), name="static")

@app.get("/")
async def serve_react_app():
    return FileResponse("react-app/build/index.html")

@app.get("/api/users")
async def get_users():
    return [{"id": 1, "name": "John Doe"}]