Base classes for the item pipeline and middleware

In the pipeline docs it says:

Each item pipeline component is a Python class that must implement the following method

I believe that it would be easier for the user and for documentation purposes to have an abstract class e.g. ItemPipeline available that raises a NotImplementedError if they forget to implement the required methods. This could also be extended to the optional methods so the user can see directly from the code which methods are supported.

This could also be applied to the downloader middleware, spider middleware and extensions.

[xPi2]

Hi, why is this still open? No decision taken?

[djunzu]

@jorenham , I would suggest to make this self registering in a separated issue/PR.