$ python test.py 
DEBUG:taskflow.engines.helpers:Looking for 'default' engine driver in 'taskflow.engines'
DEBUG:stevedore.extension:found extension EntryPoint.parse('default = taskflow.engines.action_engine.engine:SerialActionEngine')
DEBUG:stevedore.extension:found extension EntryPoint.parse('serial = taskflow.engines.action_engine.engine:SerialActionEngine')
DEBUG:stevedore.extension:found extension EntryPoint.parse('workers = taskflow.engines.worker_based.engine:WorkerBasedActionEngine')
DEBUG:stevedore.extension:found extension EntryPoint.parse('worker-based = taskflow.engines.worker_based.engine:WorkerBasedActionEngine')
DEBUG:stevedore.extension:found extension EntryPoint.parse('parallel = taskflow.engines.action_engine.engine:ParallelActionEngine')
Name: work
Type: DiGraph
Frozen: True
Nodes: 4
  - __main__.UserCreator==1.0
  - __main__.DataSourceUser==1.0
  - __main__.DataSourceGroup==1.0
  - __main__.GroupCreator==1.0
Edges: 3
  __main__.DataSourceGroup==1.0 -> __main__.GroupCreator==1.0 ({'reasons': frozenset(['group_id'])})
  __main__.DataSourceUser==1.0 -> __main__.UserCreator==1.0 ({'reasons': frozenset(['user_id'])})
  __main__.DataSourceUser==1.0 -> __main__.GroupCreator==1.0 ({'reasons': frozenset(['user_id'])})
Density: 0.250
Cycles: 0
DEBUG:taskflow.listeners.logging:Flow 'work' (7653fa11-68f9-4b5d-bcf0-6fdebcfbce25) transitioned into state 'RUNNING' from state 'PENDING'
DEBUG:taskflow.listeners.logging:Task '__main__.DataSourceUser' (80d91b54-e2e3-4d29-9bb0-77f56bd93ae4) transitioned into state 'RUNNING' from state 'PENDING'
DEBUG:taskflow.listeners.logging:Task '__main__.DataSourceGroup' (211aad71-515e-48ac-a08a-3b2b3165c708) transitioned into state 'RUNNING' from state 'PENDING'
DEBUG:taskflow.listeners.logging:Task '__main__.DataSourceUser' (80d91b54-e2e3-4d29-9bb0-77f56bd93ae4) transitioned into state 'SUCCESS' from state 'RUNNING' with result '{'user_id': 'a'}'
DEBUG:taskflow.listeners.logging:Task '__main__.DataSourceGroup' (211aad71-515e-48ac-a08a-3b2b3165c708) transitioned into state 'SUCCESS' from state 'RUNNING' with result '{'group_id': 'c'}'
DEBUG:taskflow.listeners.logging:Task '__main__.UserCreator' (2c7f0557-9d5d-4ff8-8759-2c2b012bd4fb) transitioned into state 'RUNNING' from state 'PENDING'
Creating user a
DEBUG:taskflow.listeners.logging:Task '__main__.GroupCreator' (6ba6a36d-193c-4c21-9f1f-e0e8e7f5a960) transitioned into state 'RUNNING' from state 'PENDING'
Creating user group c:a
DEBUG:taskflow.listeners.logging:Task '__main__.UserCreator' (2c7f0557-9d5d-4ff8-8759-2c2b012bd4fb) transitioned into state 'SUCCESS' from state 'RUNNING' with result 'None'
DEBUG:taskflow.listeners.logging:Task '__main__.GroupCreator' (6ba6a36d-193c-4c21-9f1f-e0e8e7f5a960) transitioned into state 'SUCCESS' from state 'RUNNING' with result 'None'
DEBUG:taskflow.listeners.logging:Flow 'work' (7653fa11-68f9-4b5d-bcf0-6fdebcfbce25) transitioned into state 'SUCCESS' from state 'RUNNING'