bork.py

w_theta = Function(V)
_, sigma_theta = split(w_theta)
theta_form = m*det(I + sigma_theta)*dx
theta = Constant(assemble(theta_form)/total_area)

def updatetheta(snes, it, rnorm, constant, w_theta):
    x = snes.getSolution()
    with w_theta.dat.vec as v:
        x.copy(v)
    constant.assign(assemble(theta_form)/total_area)
    print constant.dat.data