Week 12 of 16

FastAPI Fundamentals

From Flask to production-grade APIs — automatic validation, automatic docs, and speed that scales.

Day 56 75 minutes Watch

Day 56 of 80

Why FastAPI?

Flask taught you to think like a web developer. FastAPI teaches you to think like an API developer — and that's a different skill. Flask is excellent for learning and for rendering HTML pages. FastAPI is what production Python APIs look like in 2026.

Three things make FastAPI stand out:

Automatic validation — If a route expects an integer and receives text, FastAPI rejects it before your code even runs. You get this for free.
Auto-generated interactive docs — Visit /docs and you get a full Swagger UI where you can test every endpoint. Zero extra code.
Native async support — FastAPI routes can be async def, which unlocks the performance patterns you'll build in Week 13.

Flask vs FastAPI Side by Side

If you've been writing Flask routes, FastAPI will feel immediately familiar — but cleaner in every dimension.

What you're doing	Flask	FastAPI
Define a GET route	`@app.route("/prompts", methods=["GET"])`	`@app.get("/prompts")`
Read form/request data	`request.form` or `request.json`	Typed function parameters — no import needed
Validate incoming data	Manual — you write the checks yourself	Auto-validation via Pydantic — declare the shape, FastAPI enforces it
API documentation	None built in — you'd write it separately	Auto-generated interactive UI at `/docs`
Type hints	Optional / cosmetic	Functional — used for validation and docs generation
Async routes	Via extensions (Flask-Async)	Native — just use `async def`

Installation

Two packages: fastapi (the framework) and uvicorn (the server that runs it). Flask uses a built-in dev server; FastAPI uses uvicorn, which is production-grade from day one.

terminal bash

# Install both packages
pip install fastapi uvicorn

# Run your app (fastapi_test.py, app = FastAPI(...))
uvicorn fastapi_test:app --reload

# --reload means the server restarts when you save changes
# The server starts at http://localhost:8000

The fastapi_test:app syntax means "in the file fastapi_test.py, find the variable named app." The --reload flag is for development — never use it in production.

Watch First

Before writing any code today, watch this full tutorial. Tech With Tim covers everything from a blank file to a working API with path parameters and Pydantic models. The runtime is about 60 minutes — worth every minute.

Tech With Tim — FastAPI Full Tutorial

~60 minutes • YouTube

Watch on YouTube →

After watching, also read the official FastAPI First Steps page. It's short, well-written, and shows you the minimal viable app:

FastAPI Official Tutorial — First Steps →

Your First FastAPI App

Here's the smallest possible FastAPI application. Compare this to what Flask requires — FastAPI is more concise and does more automatically.

main.py python

from fastapi import FastAPI

app = FastAPI(title="DVP Prompt Vault API")

# @app.get means: respond to GET requests at this path
@app.get("/")
def read_root():
    return {"message": "DVP Prompt Vault API is running"}

# Path parameter: {item_id} in the URL → item_id in the function
@app.get("/items/{item_id}")
def read_item(item_id: int):
    return {"item_id": item_id}

Notice item_id: int — that type hint isn't just documentation. FastAPI uses it to validate the URL. If someone visits /items/abc, FastAPI returns a 422 error automatically. You wrote zero validation code.

HTTP Methods — The Four Verbs

Every API operation maps to an HTTP method. FastAPI has a decorator for each one:

GET — Read data. @app.get("/prompts") — fetch all prompts.
POST — Create new data. @app.post("/prompts") — add a new prompt.
PUT — Update existing data. @app.put("/prompts/{id}") — replace a prompt.
DELETE — Remove data. @app.delete("/prompts/{id}") — delete a prompt.

A well-designed REST API uses these four verbs consistently. You don't need a route called /delete_prompt — that's what DELETE /prompts/{id} is for.

The Free Gift: `/docs`

This is one of the most useful things about FastAPI. Run your app and visit http://localhost:8000/docs. You'll see a full interactive UI for every route you've defined. You can:

See all routes, grouped by tag
Click "Try it out" on any endpoint
Fill in parameters and send real requests
See the response, including status codes and response body

Zero Code Required

You didn't write any of the /docs UI. FastAPI generates it by reading your type hints and function signatures. Every time you add a route, the docs update automatically. This is the power of making type hints functional rather than cosmetic.

There's also a second docs UI at /redoc — same data, different visual style. Use whichever you prefer.

Coming Up: Pydantic Models

On Day 58 you'll use Pydantic to define exactly what shape your data should have. Here's a preview of the concept so it doesn't feel foreign when you get there:

preview — not for today python

from pydantic import BaseModel

# Define what a valid prompt looks like
class PromptCreate(BaseModel):
    platform: str
    shot: str
    prompt_text: str

# FastAPI validates the request body against this model
@app.post("/prompts")
def create_prompt(prompt: PromptCreate):
    # If the body doesn't match PromptCreate, FastAPI rejects it
    return {"message": "Created", "data": prompt}

Pydantic is installed automatically with FastAPI. You define a class, FastAPI does the rest — parsing the JSON body, validating each field, returning a clear error if anything is wrong.

Today's Goal

Watch the Tech With Tim tutorial in full and read the FastAPI First Steps page. Don't write code today — absorb the concepts. Tomorrow you'll experiment with a real working app.

End of Day Checklist

Watched the Tech With Tim FastAPI tutorial (~60 min)
Read the FastAPI First Steps official tutorial
Can explain the difference between @app.route (Flask) and @app.get (FastAPI)
Understand that type hints in FastAPI do real validation work — not just documentation
Know what /docs is and why it's generated automatically
Can name the 4 HTTP methods (GET, POST, PUT, DELETE) and what each does

Tomorrow — Day 57: Read + Experiment

You'll build fastapi_test.py — a minimal FastAPI app that serves real prompt data with GET routes and query parameters. You'll run it with uvicorn and test every endpoint in /docs.