Defining the prescribed function, f, using boundary indicators

I am attempting to define the current through a wire, which has a fairly complex geometry. In this situation the current is the prescribed function, f (using the nomenclature form the FEniCS text). Since current is a vector quantity, I must define its direction, but because the geometry is rather complex, the direction is not easily defined with a mathematical equation. So my question is this: does anyone know of a good method for defining the current using the boundary indicators of the imported mesh?

Here is essentially what I would like to do:
    1) for all locations outside of the wire subdomain, set f = 0.
    2) for all locations inside the wire subdomain, set the magnitude of f = J.
    3) for all locations where f = J, find J(hat) from a combination of the position coordinates and boundary indicator values.

My python skills are lacking, so any advice would be appreciated.

Thanks