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

Answer 2

A tiny change perimosocordiae's answer addressing the "UnboundLocalError":

def solve_sparse(A, b):
    solve_sparse.num_iters = 0

    def callback(xk):
        solve_sparse.num_iters += 1

    # call the solver on your data
    return scipy.sparse.linalg.cg(A, b, callback=callback)[0]