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Flask: Run Hello World

In order to run the Hello World application we need to set the environment variable FLASK_APP to the name of the file without the extension.

We can also set the FLASK_DEBUG environment variable to 1 tuning on the debug-mode, but this is not required for this example.

Then we execute flask run.

It is a bit different how you do this on Linux or Mac vs. on MS Windows.

In either case once Flask is running, you can use your browser to visit your new web application on the http://127.0.0.1:5000/ address or the http://localhost:5000/ address which is the same. You can also use curl on the command line to see the content of the web page.

Once you had enough enjoying the view of your new application, you can hit Ctr-C to stop the program.

Linux/Mac:

$ export FLASK_APP=app
$ export FLASK_DEBUG=1
$ flask run

or

FLASK_APP=app FLASK_DEBUG=1 flask run

Then visit: http://127.0.0.1:5000/ or type in

curl http://localhost:5000/

MS Windows

On the command line or in the terminal of PyCharm, VS Code or any other IDE:

set FLASK_APP=app
set FLASK_DEBUG=1
flask run

Stop the application

To stop the application press Ctrl-C,

Troubleshooting

Address already in use

When you try to run your application you might encounter this error

$ FLASK_APP=app FLASK_DEBUG=1 flask run
 * Serving Flask app 'app'
 * Debug mode: on
Address already in use
Port 5000 is in use by another program. Either identify and stop that program, or start the server with a different port.

This means that there is another application running on the same port. It might be that you have already tried flask in a different window and you have not shut it down.

On Linux or Mac it might be that you used Ctrl-Z to stop the program. Which actually only suspends it, but keeps the port used.

The solution is to either find the other instance and close it using Ctrl-C or to launch this instance on a different port.

You can do the latter by provideing the --port parameter. e.g.

$ FLASK_APP=app FLASK_DEBUG=1  flask run --port 8080
  • FLASK_DEBUG
  • FLASK_APP
  • run
  • port