LightShadows.Soft  

// Set light to cast 4x filtered soft shadows (disc)

light.shadows = LightShadows.Soft;

using UnityEngine;
using System.Collections;

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

import UnityEngine
import System.Collections

class example(MonoBehaviour):

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