For GPL works the answer is quite simple - it depends on what the author of the program (like Wordpress) defines as an internal or external interface. If you are linking to an interface that is not external, then you are making your code part of the original program and thus creating a derivative work.
Posted Jul 29, 2010 16:21 UTC (Thu) by njs (guest, #40338)
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I've never seen a legal test for derivative works that made reference to whether some interface was declared "external" or "internal". So, citation needed, I guess.
WordPress, themes, and derivative works
Posted Jul 29, 2010 19:41 UTC (Thu) by smitty_one_each (subscriber, #28989)
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Here is an idea which is at least testable: define 'derivative work' to mean the part which cannot be ported to another platform.
So you go to, oh, cherry.py, and make your theme work.
Inevitably, there would need to be some python shimming to de-conflict your namespaces, make up for missing functionality, and the like.
The part of your work that runs the same everywhere is called the 'theme'.