FastAPI - Hello World

The requirements include both FastAPI and pytest so we can write and run tests for our web application.

It is recommended to set up a virtual environment or use some other way to separate environments.

virtualenv -p python3 venv
source venv/bin/activate
pip install -r requirements.txt

We need a single file in which we import the FastAPI class.
We then create an instance of it. We can call it any name, but most of the examples are using app so we'll use that as well.
For each URL path we need to create a mapping to indicate which function needs to be executed when someone access that path.
The functions are defined as async so FastAPI can handle several requests at the same time. The name of the function does not matter.
The decorator @app.get("/") is doing the mapping.
The function needs to return some Python data structure that will be converted to JSON when it is sent back to the client.

from fastapi import FastAPI

app = FastAPI()


@app.get("/")
async def root():
    return {"message": "Hello World"}

In order to see this working launch the development web server that comes with the installation of FastAPI.

fastapi dev main.py

Then visit http://localhost:8000/

You can also visit some other pages on this site: