Expression is evaluated at interior points even if it occurs as boundary integral only

An expression which is part of a boundary integral in a form gets unnecessarily evaluated at interior mesh points when assembling. This is problematic as the Expression may be very expensive to evaluate or may not be even defined at these points.

Also note that the form must contain at least one volume part for this problem to occur.

Following code demonstrates the problem:

from dolfin import *

mesh = UnitIntervalMesh(10)
V = FunctionSpace(mesh, "CG", 1)
class ExpensiveExpression(Expression):
    def eval(self, value, x):
        print "Evaluating an expensive expression at x = %f." % x
        value[0] = 1

f0 = ExpensiveExpression()
f = interpolate(Constant(1), V)

dx = dx(domain=mesh)
ds = ds(domain=mesh)

form = f * dx + f0 * ds
assemble(form)

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