Abc meta classes

[jlvdb]

This pull requests adds abc.ABC as meta classes for Pipeline and PipelineStage.

Both classes implement a few abstract methods, however they do not use abc.ABC as meta class, which would prevent to instantiate a subclass if it does not properly implement the abstract methods. Currently a NotImplementedError is raised at a later point during runtime.

[jlvdb]

What is the purpose of the abstract method here? Clearly the issue I tried to address is a design feature, but I was not able to figure it out.

[joezuntz]

Hi @jlvdb - thanks for looking at this! I'm not quite clear what you mean by your question - could you explain more?

[jlvdb]

Hi @joezuntz, I did not really understand this line here:

ceci/ceci/stage.py

Line 293 in 5c683f8

and not getattr(cls.run, "__isabstractmethod__", False)

Since the it checks for isabstractmethod, I thought that the @AbstractMethod serves a purpose beyond indicating an abstract method that should be overwritten, since suddenly the unit tests were failing.

[joezuntz]

I don't really remember the details here, but I think the main reason I didn't go full-on with using the ABC superclass was that the error messages were much less informative for a typical user. So I guess I was manually using this test to make sure that the user overrode stuff so I could give a more useful custom error message.