You can create and run a minimal Flask application with one Python file: install Flask in a project-specific virtual environment, define an app and route, then start Flask’s local development server. The steps below work on macOS, Linux, and Windows; deployment requires a production server rather than Flask’s development server.
What you need before creating a Flask app
Install Python 3.9 or newer; Flask’s current installation guide recommends using the latest Python version. Use a virtual environment so this project’s packages stay separate from system packages and other projects. See Flask’s installation guide for platform-specific details.
Create and run a minimal Flask application
- Create a project directory and enter it. For example, create a folder named
flask-quickstartand open a terminal in that folder. - Create a virtual environment. On macOS or Linux, run
python3 -m venv .venv. On Windows, runpy -3 -m venv .venv. - Activate the environment. On macOS or Linux, run
. .venv/bin/activate. In Windows Command Prompt, run.venvScriptsactivate. - Install Flask. Run
pip install Flask. - Create
hello.pywith this code:from flask import Flask app = Flask(__name__) @app.route("/") def hello_world(): return "<p>Hello, World!</p>" - Start the local server. Run
flask --app hello run. - Open the address Flask prints. It is normally
http://127.0.0.1:5000/. The page should display “Hello, World!”
In Flask(__name__), the module name helps Flask locate resources such as templates and static files. The @app.route("/") decorator connects the root URL to hello_world, whose return value becomes the response. Flask’s Quickstart explains these core pieces and the run command.
Avoid a module-name conflict
Do not name your Python file flask.py; that name can conflict with the installed Flask package. The example uses hello.py. If you instead call the file app.py or wsgi.py, Flask can discover the application without an explicit --app argument in common cases.
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Choose a structure that fits the project
A single module is a sound way to get a small app responding quickly. Flask does not require one particular project layout. When the project gains routes, templates, or tests, a package structure can make its parts easier to manage. Flask’s project layout tutorial shows one organized approach.
| Approach | Setup speed | As the project grows | Testing and configuration |
|---|---|---|---|
| One module | Fastest: define the application and route in one file. | Can become harder to organize as features accumulate. | Simple for a demonstration; no factory pattern is needed. |
| Package with an application factory | Requires more initial organization. | Separates application code and supports a more structured project. | A create_app function can create separately configured instances, including for tests; see Flask’s application setup tutorial and application factories guide. |
Start with one file if the goal is simply to learn how a route works. Consider a package and factory when you need clearer separation or multiple app configurations; neither is a prerequisite for running Flask.
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Keep local development separate from deployment
flask --app hello run starts Flask’s built-in development server. It is intended for local development, not public production use: Flask says the server is not designed to be particularly secure, stable, or efficient. Debug mode can reload code and provide an interactive debugger, but that debugger can execute Python code, so never expose it in production. For deployment, use a dedicated WSGI server or a hosting platform, as described in Flask’s production deployment guide.
The official tutorial demonstrates packaging an application as a wheel, installing it in another environment, replacing its example SECRET_KEY='dev' with a random secret, and using Waitress. Its waitress-serve --call 'flaskr:create_app' command is specific to the tutorial’s flaskr factory application; it is not a universal command for every Flask project. Follow the instructions for the server and application structure you actually deploy. See the tutorial’s deployment walkthrough for that example.
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