VisTrails
Break down the List type
Right now there is a single List type that can represent different things. It represents Python lists (potentially nested), but also numpy arrays (which don't always behave like list, for instance + doesn't concatenate, and truth value has a different meaning), and it's also the magic type for streaming and aggregating multiple connections (i.e. it is special to port depth).
This is an attempt to rework that. It introduces Array, which is the base class (basically replaces List), plus two subclasses, List (that now represents a Python list, a module declaring this port type can expect a proper Python list) and NumpyArray (for a numpy.ndarray).
This came up during my work on the interpreter and pandas.
It turned out to be complicated, because of all the code that checks for the List type (for depth magic). I'm also not sure whether List shouldn't completely be eliminated (though NumpyArray is definitely useful).
Would it not be better to only do the list magic on List, not Array, so that it is not possible to accidentally loop a numpy array? Looping a list over a module, or aggregating connections, usually requires a Python list.
We could also get rid of List by converting it to a Variant of depth 1. The looping magic already treats them the same. In most cases it is better to use a typed list (type of depth 1) for a simple list of objects, or a numpy array for bigger data.
Would it not be better to only do the list magic on List, not Array
This is my intention, but Array ports have to accept what List ports accept, which is not the case right now.