Currently, you can do something like this: Task(Flow/RunID/StepName) and this will not result in an error but then the resulting Task object behaves in a bizarre manner where things like t.data will work but t.data.my_artifact will not for example.

We should validate the format of the pathspec passed in to each object and verify that the following are the only possible cases:

• Metaflow()
• Flow('MyFlow')
• Run('MyFlow/1')
• Step('MyFlow/1/start')
• Task('MyFlow/1/start/1')
• DataArtifact('MyFlow/1/start/1/name')

We should make sure that MyFlow and MyFlow/ behave similarly for example though.