Development Server

Starting with Flask 0.11 there are multiple built-in ways to run a development server. The best one is the flask command line utility but you can also continue using the method.

Command Line

The flask command line script (Command Line Interface) is strongly recommended for development because it provides a superior reload experience due to how it loads the application. The basic usage is like this:

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

This will enable the debugger, the reloader and then start the server on http://localhost:5000/.

The individual features of the server can be controlled by passing more arguments to the run option. For instance the reloader can be disabled:

$ flask run --no-reload


The alternative way to start the application is through the method. This will immediately launch a local server exactly the same way the flask script does.


if __name__ == '__main__':

This works well for the common case but it does not work well for development which is why from Flask 0.11 onwards the flask method is recommended. The reason for this is that due to how the reload mechanism works there are some bizarre side-effects (like executing certain code twice, sometimes crashing without message or dying when a syntax or import error happens).

It is however still a perfectly valid method for invoking a non automatic reloading application.