Retrieving number of iterations that ran for sparse linear solver in SciPy

Question

How to retrieve how many iterations ran to achieve specified tolerance level in SciPy sparse linear system solvers?

Answer 1

For Python 3, the following works:

    def solve_sparse(A, b):
      num_iters = 0

      def callback(xk):
         nonlocal num_iters
         num_iters+=1

      x,status=scipy.sparse.linalg.cg(A, b,tol=1e-15, callback=callback)
      return x,status,num_iters