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
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 CLI command is the recommended way to run the development server. Use
--app option to point to your application, and the
--debug option to enable
$ flask --app hello run --debug
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
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. You can choose to disable this service instead of using a different port by searching for “AirPlay Receiver” in System Preferences and toggling it off.
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.
The development server can also be started from Python with the
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__":
$ python hello.py