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  • Documentation version: 1.1.x

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 Flask.run() 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_ENV=development
$ flask run

This enables the development environment, including the interactive debugger and reloader, and then starts 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


Prior to Flask 1.0 the FLASK_ENV environment variable was not supported and you needed to enable debug mode by exporting FLASK_DEBUG=1. This can still be used to control debug mode, but you should prefer setting the development environment as shown above.

In Code

The alternative way to start the application is through the Flask.run() 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.