How to define Dirichlet boundary conditions using patches?

When we use hip meshes, we have information about boundaries stored in patches (check out How to convert a CERFACS mesh? to find out how to load this information in FEniCS). Defining Dirichlet boundary conditions using these patches is very similar to the case where we use a function to identify the boundary (see this example), but instead we pass the boundary markers and the number corresponding to the patch.

from dolfin.fem.dirichletbc import DirichletBC

bc = DirichletBC(V, u_D, boundary_markers, 0)

The index is given by the ordering of patch labels.