I do not seem to understand how to import modules into an apache airflow DAG definition file. I would want to do this to be able to create a library which makes declaring ta
Are you using Airflow 1.9.0? This might be fixed there.
The issue is caused by the way Airflow loads DAGs: it doesn't just import them as normal python modules, because it want's to be able to reload it without restarting processes. As a result . isn't in the python search path.
If 1.9.0 doesn't fix this, the easiest change is to put export PYTHONPATH=/home/airflow/airflow/:$PYTHONPATH in the startup scripts. The exact format of that will depend on what you are using (systemd vs init scripts etc.)