How can I use DirichletBC.gather()?

Question

Dear FEniCS person,
I am running an application in parallel and wondering how I can get the global set of boundary values for an object of type DirichletBC.

I test with the following script

global_bc.py

from dolfin import *
m = UnitCubeMesh(1,1,1)
V = FunctionSpace(m, "CG", 1)
bc = DirichletBC(V, 1.0, "near(x[0], 0.0)")
bc.gather()
bc_dofs = bc.get_boundary_values().keys()
print bc_dofs

If i run the script
mpirun -n 3 python global_bc.py

I get the output
[]
[2L, 4L, 5L, 7L]
[]

Which looks like only the second processor owns the dofs in the bc. The method gather() returns a dictionary and accepts no parameters so I am confused as to how it should be used if not as in the script.

Best Regards,
Gabriel

Answer 1

This looks like a bug in the Python wrapping of gather.

The function gather is used by DirichletBC to distribute shared boundary values between processes. However, the C++ version takes the boundary values as argument, checks if the bv dof is shared and in that case the bv is distributed to its neighbours. This does not work from the Python since you're not allowed to send the boundary values in as argument (the Python gather takes no arguments). You should report the bug on bitbucket.