First Flask Application (Hello World)

This is the smallest useful Flask app.

Create app.py

python

from flask import Flask
 
app = Flask(__name__)
 
 
@app.route("/")
def home():
    return "Hello, Flask!"

What’s happening?

• app = Flask(__name__) creates the Flask application object.
  • __name__ helps Flask know where to find templates/static files later.
• @app.route("/") registers a route.
  • When a request hits /, Flask calls home().
• Returning a string becomes an HTTP response body.

Run it (two common ways)

Option A: flask run (recommended)

Set the Flask app entry point:

bash

export FLASK_APP=app
flask run

Option B: python app.py

You’ll see this pattern in many tutorials:

python

if __name__ == "__main__":
    app.run(debug=True)

This is valid, but for this tutorial track we’ll mainly use flask run because it matches production patterns better.

Common beginner error

If you see “Working outside of application context” later, it’s usually because something that expects app is being used without a request/app context.

Don’t worry about it now—just remember Flask has context concepts.

Visualize it

Here’s how a single request flows through the WSGI server and your Flask app before a response comes back.

diagram WSGI request lifecycle mermaid

  flowchart LR
  B["Browser"] --> W["WSGI server"]
  W --> F["Flask app"]
  F --> M["Matched view function"]
  M --> R["Response"]
  R --> B

How a browser request travels through Flask and back

🧪 Try It Yourself

Exercise 1 – Create a Flask App

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Exercise 2 – Dynamic Route

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Exercise 3 – Return JSON

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