Under suitable scaling, the structure of self-gravitating polytropes is described by the standard Lane-Emden equation (LEE), which is characterised by the polytropic index n. Here we use the known exact solutions of the LEE at n = 0 and 1 to solve the equation perturbatively. We first introduce a scaled LEE (SLEE) where polytropes with different polytropic indices all share a common scaled radius. The SLEE is then solved perturbatively as an eigenvalue problem. Analytical approximants of the polytrope function, the radius and the mass of polytropes as a function of n are derived. The approximant of the polytrope function is well-defined and uniformly accurate from the origin down to the surface of a polytrope. The percentage errors of the radius and the mass are bounded by 8.1 × 10 −7 per cent and 8.5 × 10 −5 per cent, respectively, for n ∈ [0, 1]. Even for n ∈ [1, 5), both percentage errors are still less than 2 per cent.