How can I debug Openstack Dashboard?

As you run server by runserver command you should go to the root url like 127.0.0.1:8000/ not 127.0.0.1:8000/horizon

The difference is in running method. When you installed openstack_dashboard there is apache config that listen to address http://host/horizon look in to /etc/apache2/conf/sites-available/

The directories

/usr/lib/python2.7/dist-packages/horizon/

Is like the framework for all dashboards

/usr/share/openstack-dashboard/

Is the default dashboard for openstack

/etc/openstack-dashboard/local_settings.py

The config file for dashboard. Here you can apply changes for default dash, for example attach your project that modifies or extends default dash

So, your problem will be solved if you follow that link http://127.0.0.1:8000/ For debugging use

pdb or ide with integrated debugger

Follow this link. http://docs.openstack.org/developer/horizon/quickstart.html

Once this is done, add pdb statements to your code that you wish to debug.Go into the horizon folder and run the following command ./run_tests.sh --runserver 127.0.0.1:port_number

Instead of local host you can put in the IP address where horizon is running

I have written the OpenStack documentation for debugging dashboards using PyCharm.
(It is good for any Openstack Dashboard including Horizon) http://docs.openstack.org/developer/mistral/developer/troubleshooting.html