Is there a way to disapply/remove BCs?

Question

Suppose we have this timoshenko beam
https://bitbucket.org/jackhale/timoshenko/src/30f59208921c339f0d94a1272e5b53f337df2c8f/cantilever.py?at=master

But instead of applying the bcs in the for loop in line 72-76 we do

A_matrix, b_vector = assemble_system(A, L, bcs)

I want the reaction forces on left_boundary. For that I can do

reaction = Function(U)
reaction.vector()[:] = A_matrix * u_h.vector()

But that will give me 0 on the boundary point.
If I reassemble A_matrix without the bcs it gives the correct result as can be seen doing

reaction = Function(U)
reaction.vector()[:] = A_matrix * u_h.vector()
reaction_forces = reaction.split(deepcopy=True)[1]
plot(reaction_forces); interactive()
AA_matrix = assemble(A)
reaction.vector()[:] = AA_matrix * u_h.vector()
reaction_forces = reaction.split(deepcopy=True)[1]
plot(reaction_forces); interactive()

But if the assembly takes a long time I don't want to do it again. Is there a way to operate A_matrix without the bcs or to remove/detach/disapply them?

I've put a working example here http://pastebin.com/b0QenJWd

Comments

I'm not quite sure what you're trying to accomplish, but you can always create a copy of A_matrix:

A_matrix = assemble(A)
AA_matrix = A_matrix.copy()

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I can always copy, but I get it without the BCs only if it was assembled with assemble(a) and if I do it before applying said BCs.
If I use assemble_system() I never get to access the matrix without the BCs applied.

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Undoing the addition of boundary conditions is not possible without reassembling the missing stencils. So in FEniCS it probably makes most sense to create a copy where necessary. And yes, if you use utilities like assemble_system you end up with a constrained system. So the solution would be not to do this and set up the system step by step.

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Thank you. Should I delete this question since it has no solution? (Meaning, get stuff without BCs after assemble_system). Or you could put your comment in an answer to be accepted.