LightShadows.Soft  

// Set light to cast 4x filtered soft shadows

light.shadows = LightShadows.Soft;

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void Awake() {
light.shadows = LightShadows.Soft;
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def Awake():
light.shadows = LightShadows.Soft