FastAPI is a modern Python framework for building APIs, which are the interfaces that let different software applications talk to each other. It's designed to be fast, easy to use, and robust.

It has several key features that make it a popular choice for developers:

• High Performance: It's one of the fastest Python frameworks available, on par with NodeJS and Go, thanks to its foundation on Starlette (for the web parts) and Pydantic (for the data parts).
• Fast to Code: Features like type hints and automatic documentation speed up development significantly.
• Automatic Docs: It automatically generates interactive API documentation. You don't have to write a single extra line of code to get a user interface where you can test your API.
• Data Validation: It uses standard Python type hints to validate incoming data. If a request sends the wrong type of data, FastAPI automatically rejects it with a clear error message.

To start using FastAPI, you need to install two packages. The first is the framework itself. The second is a server to run your application, called Uvicorn.

Uvicorn is an ASGI server, which stands for Asynchronous Server Gateway Interface. It's the component that actually receives requests from the internet and passes them to your FastAPI application.

Let's create a simple API. Create a file named main.py and add the following code.

Here’s what’s happening:

1. from fastapi import FastAPI: We import the necessary class from the library.
2. app = FastAPI(): We create an instance of our application.
3. @app.get("/"): This is a decorator. It tells FastAPI that the function right below it, read_root, is in charge of handling requests that go to the main URL (/) using a GET method.
4. return {"Hello": "World"}: We return a Python dictionary. FastAPI automatically converts this into JSON format for the response.

To run this application, open your terminal in the same directory as your main.py file and run this command:

This command tells Uvicorn to run the app object from your main.py file. The --reload flag makes the server restart automatically whenever you save changes to the file. Now, open your web browser and go to http://127.0.0.1:8000. You should see the JSON response: {"Hello":"World"}.

One of FastAPI's most powerful features is its automatically generated documentation. With your server still running, navigate to http://127.0.0.1:8000/docs in your browser. You'll find an interactive page that documents your API.

This interface, known as Swagger UI, allows you or anyone else using your API to see all the available endpoints, their parameters, and even test them directly from the browser. This is generated automatically from your code and type hints, saving you the work of manually writing documentation.

Now let's check your understanding of these core concepts.