ScriptableObject.OnEnable  

function OnEnable () {
    print("script was enabled");
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void OnEnable() {
        print("script was enabled");
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def OnEnable():
        print('script was enabled')