Development Server

Flask provides a run command to run the application with a development server. In debug mode, this server provides an interactive debugger and will reload when code is changed.

Warning

Do not use the development server when deploying to production. It is intended for use only during local development. It is not designed to be particularly efficient, stable, or secure.

See Deploying to Production for deployment options.

Command Line

The flask run CLI command is the recommended way to run the development server. Use the --app option to point to your application, and the --debug option to enable debug mode.

$ flask --app hello --debug run

This enables debug mode, including the interactive debugger and reloader, and then starts the server on http://localhost:5000/. Use flask run --help to see the available options, and Command Line Interface for detailed instructions about configuring and using the CLI.

Address already in use

If another program is already using port 5000, you’ll see an OSError when the server tries to start. It may have one of the following messages:

  • OSError: [Errno 98] Address already in use

  • OSError: [WinError 10013] An attempt was made to access a socket in a way forbidden by its access permissions

Either identify and stop the other program, or use flask run --port 5001 to pick a different port.

You can use netstat or lsof to identify what process id is using a port, then use other operating system tools stop that process. The following example shows that process id 6847 is using port 5000.

$ netstat -nlp | grep 5000
tcp 0 0 127.0.0.1:5000 0.0.0.0:* LISTEN 6847/python

macOS Monterey and later automatically starts a service that uses port 5000. To disable the service, go to System Preferences, Sharing, and disable “AirPlay Receiver”.

Deferred Errors on Reload

When using the flask run command with the reloader, the server will continue to run even if you introduce syntax errors or other initialization errors into the code. Accessing the site will show the interactive debugger for the error, rather than crashing the server.

If a syntax error is already present when calling flask run, it will fail immediately and show the traceback rather than waiting until the site is accessed. This is intended to make errors more visible initially while still allowing the server to handle errors on reload.

In Code

The development server can also be started from Python with the Flask.run() method. This method takes arguments similar to the CLI options to control the server. The main difference from the CLI command is that the server will crash if there are errors when reloading. debug=True can be passed to enable debug mode.

Place the call in a main block, otherwise it will interfere when trying to import and run the application with a production server later.

if __name__ == "__main__":
    app.run(debug=True)

$ python hello.py