Relative Shapekeys (Relative Keys)

Is it possible to change the relative key of relative shape keys in the python API? The Blender default is the Basis key. I cannot find a way to modify this inside the API, which makes it difficult to model linear deformation.

I have 1000’s of complex shape keys and would like to morph from one shape to the next in a linear manner. Absolute shape keys seems to give rise to strange results.