[SciPy-User] preconditioner for GMRES

I am developping a Jacobian-Free Newton-Krylov code to solve the Navier Stokes equations. In this code, the product of the Jacobian matrix with a given vector is represented by the matvec method of a "LinearOperator"object. The scipy.sparse.linalg.isolve.gmres method can takes an optional preconditioner as parameter which is either an object "LinearOperator" or a matrix. However, the scipy documentation does not explain how one can form this preconditioner in a (ideally) matrix-free manner given only the matvec method of the jacobian. Any suggestions on this would be very appreciated. Thanks, Christine