Hi,
As your error suggest, the last argument has to be a FunctionSpace. You are supplying a FiniteElement. Thus by supplying W.sub(0).collapse() the interpolation will work. However, Since you are working with a MixedSpace, i would suggest using a FunctionAssigner for assigning the solutions to w. See: https://fenicsproject.org/docs/dolfin/1.6.0/python/programmers-reference/cpp/function/FunctionAssigner.html#dolfin.cpp.function.FunctionAssigner