FASTAPI FUNDAMENTALS

1. What Is FastAPI?

FastAPI is a modern, high‑performance Python framework (Python 3.7+) that uses type hints for data validation and leverages two proven libraries:

Component	Purpose
Starlette	Handles routing, websockets, middleware, and ASGI‑compatibility.
Pydantic	Validates request/response schemas via dataclasses (`BaseModel`).

Why it matters

Speed – FastAPI is one of the fastest Python web frameworks; it rivals or out‑performs JavaScript stacks like Express.js.
Developer ergonomics – Writing an endpoint is a matter of a single function, and the type hints give instant IDE support and runtime checks.
Docs magic – An interactive Swagger UI (/docs) and ReDoc (/redoc) are auto‑generated from your code.

Code snippet – instantiating the appfrom fastapi import FastAPI app = FastAPI()

ComparisonExpress.js (Node) and FastAPI both expose quick GET/POST routes, but FastAPI brings Python‑style type hints to the table.

2. Setting Up the Environment

Step	What to do	Why it matters
Virtual env	`python -m venv .venv`	Keeps your project’s packages isolated from the global Python installation.
Activate	`source .venv/bin/activate` (Unix) or `.venv\Scripts\activate` (Windows)	Turns the virtual env on for the current shell session.
Install core libs	`pip install fastapi uvicorn`	`uvicorn` is a lightweight ASGI server that keeps the app running in dev mode.
Run the server	`uvicorn main:app --reload`	`main` = Python file name, `app` = FastAPI instance; `--reload` restarts automatically on code changes.
IDE	VS Code (demo used this)	Any editor works, but VS Code supports Python linting and FastAPI plugins.

3. Basic GET Endpoints – “Hello World” to Dynamic Paths

Goal	Explanation	Minimal Code
Root	Return a friendly JSON payload.	`python\[email protected]("/")\ndef read_root():\n return {"message": "Hello World"}\n`
Path param	Pull a value from the URL itself; FastAPI casts it based on your type hint.	`python\[email protected]("/user/{user_id}")\ndef read_user(user_id: int):\n return {"user_id": user_id}\n`
Query param	Optional data appended after a `?` token; you can give defaults or set `Optional`.	`python\[email protected]("/user")\ndef get_user(limit: int = 10, name: Optional[str] = None):\n return {"limit": limit, "name": name}\n`
Conditional logic	Show how you can tailor the response depending on a query string.	`python\[email protected]("/numbers")\ndef list_numbers(limit: int = 0):\n if limit:\n return list(range(limit))\n return list(range(100))\n`

Practical tip – If you navigate to http://localhost:8000/docs, the Swagger UI will let you test each of these endpoints directly in the browser.

4. POST Requests & Pydantic Models

FastAPI automatically transforms incoming JSON into a Pydantic model, validates it, and returns it back to you.

Step	Explanation	Code
Create a model	Describes the shape and types of your data.	`python\nclass User(BaseModel):\n id: int\n name: str\n age: int\n`
POST endpoint	Accepts a `User` instance as its body.	`python\[email protected]("/create_post")\ndef create_user(user: User):\n # Normally you’d persist` user `to a DB\n return user\n`
Docs	Visiting `/docs` will show a form where you can enter a JSON payload; it will also flag missing required fields.