inp = self.GetInput();
out = self.GetOutput();
out.ShallowCopy(inp);

scalarFieldDataValue = 0.1
scalarFieldDataName = "foo"

ref = out

if not ref is None:
  value = vtk.vtkFloatArray();
  value.SetName(scalarFieldDataName);
  value.SetNumberOfComponents(1);
  value.InsertNextValue(scalarFieldDataValue);
  ref.GetFieldData().AddArray(value);