Print current residual from callback in scipy.sparse.linalg.cg

I am using scipy.sparse.linalg.cg to solve a large, sparse linear system, and it works fine, except that I would like to add a progress report, so that I can monitor the residual as the solver works. I've managed to set up a callback, but I can't figure out how to access the current residual from inside the callback. Calculating the residual myself is possible, of course, but that is a rather heavy operation, which I'd like to avoid. Have I missed something, or is there no efficient way of getting at the residual?