Flask provides a
run command to run the application with a
development server. In development mode, this server provides an
interactive debugger and will reload when code is changed.
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.
flask run command line script is the recommended way to run the
development server. It requires setting the
variable to point to your application, and
fully enable development mode.
$ export FLASK_APP=hello $ export FLASK_ENV=development $ flask run
$ set -x FLASK_APP hello $ export FLASK_ENV=development $ flask run
> set FLASK_APP=hello > set FLASK_ENV=development > flask run
> $env:FLASK_APP = "hello" > $env:FLASK_ENV = "development" > flask run
This enables the development environment, including the interactive
debugger and reloader, and then starts the server on
flask run --help to see the available
options, and Command Line Interface for detailed instructions about configuring
and using the CLI.
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
Address already in use¶
If another program is already using port 5000, you’ll see an
when the server tries to start. It may have one of the following
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
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
$ lsof -P -i :5000 Python 6847 IPv4 TCP localhost:5000 (LISTEN)
> netstat -ano | findstr 5000 TCP 127.0.0.1:5000 0.0.0.0:0 LISTENING 6847
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”.
Lazy or Eager Loading¶
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.
This feature is called “lazy loading”.
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.
To override this behavior and always fail immediately, even on reload,
--eager-loading option. To always keep the server running,
even on the initial call, pass
As an alternative to the
flask run command, 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 the debugger and reloader, but
FLASK_ENV=development environment variable is still required to
fully enable development 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