@jacktat why are you sending in a list (surf_inlet)? This should just be the integer in the list,Which causes the error. You should call this constructor
dolfin.cpp.fem.DirichletBC(V: dolfin.cpp.function.FunctionSpace, g: dolfin.cpp.function.GenericFunction, sub_domains: dolfin.cpp.mesh.MeshFunctionSizet, sub_domain: int, method: str = ‘topological’)