Flask Hello World

1 Introduction to Flask

Flask is a lightweight WSGI web framework for Python. It gives you the essentials—routing, request/response handling, templating (via Jinja2), and a solid extension ecosystem—without the bulk of a full-stack platform.

Why people reach for Flask:

Minimal core with batteries-available extensions.
Flexible project layout—from a single file to modular apps.
Readable and approachable codebase.
Production-ready when paired with a WSGI server like Gunicorn.

Click "Next" to set up your environment.

2 Prerequisites: Python & pip

Install Python 3.8 or newer, then confirm with:

python --version

Need Python? Grab it from python.org.

3 Create a virtual environment

Isolate dependencies with a virtual environment.

python -m venv .venv

Activate it:

# macOS/Linux
source .venv/bin/activate

# Windows (Command Prompt)
.venv\Scripts\activate

You’ll see (.venv) in your prompt when active.

4 Install Flask

With your virtual environment active, install Flask.

pip install Flask

Confirm the install:

pip list | grep -i flask

5 Write a minimal Flask app

Create a file named app.py with this content:

from flask import Flask

app = Flask(__name__)

@app.get("/")
def hello():
    return "Hello from Flask!"

@app.get("/hello/")
def hello_name(name: str):
    # URL converters: string, int, float, path (default is string)
    return f"Hello, {name}!"

if __name__ == "__main__":
    # For local development only
    app.run(host="127.0.0.1", port=5000, debug=True)

Route decorators map HTTP methods to Python functions. The angle-bracket segment captures a path parameter.

6 Run the app

Start the development server:

python app.py

You should see output pointing at http://127.0.0.1:5000. The terminal will block; stop it with Ctrl+C.

7 Test the endpoints

Open your browser:

http://127.0.0.1:5000/ — “Hello from Flask!”
http://127.0.0.1:5000/hello/Flask — “Hello, Flask!”

You can also use curl:

curl http://127.0.0.1:5000/
curl http://127.0.0.1:5000/hello/World

8 Use Flask’s debug mode

In development, debug=True enables live code reload and better error traces—very helpful when iterating.

If you didn’t enable it earlier, change the last lines in app.py:

if __name__ == "__main__":
    app.run(host="127.0.0.1", port=5000, debug=True)

Note: do not use the built-in server in production. For production, run behind a real WSGI server (e.g., Gunicorn) behind a reverse proxy (e.g., Nginx).

9 Project structure (optional)

As projects grow, a tidy layout helps:

hello-flask/
├─ app/
│  ├─ __init__.py
│  └─ routes.py
├─ requirements.txt
└─ run.py

Example app/__init__.py:

from flask import Flask

def create_app():
    app = Flask(__name__)
    from .routes import bp
    app.register_blueprint(bp, url_prefix="/")
    return app

Example app/routes.py:

from flask import Blueprint

bp = Blueprint("main", __name__)

@bp.get("/")
def hello():
    return "Hello from Flask!"

@bp.get("/hello/")
def hello_name(name: str):
    return f"Hello, {name}!"

Example run.py:

from app import create_app

app = create_app()

if __name__ == "__main__":
    app.run(host="127.0.0.1", port=5000, debug=True)

Freeze dependencies with pip freeze > requirements.txt.

10 Next steps and common pitfalls

Templates: use render_template("index.html", name=name) with Jinja2 files.
Static files: serve CSS/JS from a static/ folder.
Configuration: keep secrets out of code; use environment variables.
Production: run behind Gunicorn or uWSGI, behind a reverse proxy.
Debug server won’t reload on Windows with git-bash + virtualenv—use Command Prompt or PowerShell to activate the venv there.

You now have a working Flask app. From here you can add templates, forms, and a real project layout without changing the core idea.

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