The Hello World, Line by Line🔖

Let's read the minimal Flask program the way you'd read any unfamiliar code — one line at a time, asking "what does this do and why?"

The import🔖

from flask import Flask

flask (lowercase) is the library. Flask (uppercase) is the class.

"But those are the same word!" — almost. Python is case sensitive. There's also a convention at work: libraries are lowercase, classes are capitalized. This isn't a language rule, just a practice that Python programmers follow.

Think of lowercase flask as a huge collection of LEGO. Capital Flask is a specific box from that collection — the one that builds the main robot. All the other pieces in the flask library extend and build on that robot.

This line opens the collection and pulls out the main box.

Building the app🔖

app = Flask(__name__)

This line builds the LEGO robot from the Flask kit. In OOP terms, it calls the constructor — the function that creates a new Flask object.

Now we have a Flask robot named app. Every time you see app.something in the code, we're talking to this particular robot — calling one of its methods or reading one of its properties.

What's __name__? Python has internal variables identified by double underscores. __name__ is a flag that identifies how this Python file is being run. For now, think of it as telling Flask "the name of this program." The details don't matter yet.

app is just a variable name. It's convention, not magic. But every Flask tutorial uses it, so we follow convention.

The route decorator🔖

@app.route("/")
def index():
    return "<h1>Hello, World!</h1>"

This is three things at once. Let's unpack from the inside out.

The path: "/"🔖

/ is the root path. Remember, URLs have the structure:

computer:port/resource

So localhost:5000/ means "the root resource." Instead of requesting a specific file, requesting / is like knocking on someone's door and saying "Hey, whaddaya got?"

Even if you have nothing else to share, you have something at /.

The method: .route("/")🔖

.route() is a method that belongs to our app object. It's the same dot-method pattern you've used before — like my_list.sort() or random.randint().

Somewhere inside the Flask library, there's code that defines how .route() talks to the HTTP server. We don't need to read that code. We just need to know that our robot has a .route() superpower that connects it to the HTTP world.

The decorator: @🔖

The @ symbol is a Python decorator. Decorators mark a chunk of code that should run at a particular moment, rather than top-to-bottom as Python reads the file.

@app.route("/") is a trigger — like event handlers in CMU Graphics. It says:

"When the Flask app receives an HTTP request matching /, run the function directly below this line."

The function doesn't run when Python reads it. It runs when a browser asks for that URL.

The function🔖

def index():
    return "<h1>Hello, World!</h1>"

A plain Python function. It returns a string. That string goes back through Flask, back to the HTTP server, back through the network, and arrives at the browser that made the request.

The function name index is convention — it matches the / route, like index.html is the default page on a web server. But the name could be anything. Flask cares about the decorator, not the function name.