killing a rosrun process using python script

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Hi, I am writing a GUI interface to run nodes in ROS. The following code execute a rosrun process when pressing Button1. I looked for a code in order to kill that process when pressing Button 2. I did not find something specific or related to this work.

#simple GUI

import os, time, signal, threading
from Tkinter import * 
from subprocess import call
from subprocess import Popen  
import subprocess
import numpy as np
import cv2
#from PIL import Image, ImageTk
from PIL import *


def f1():
    subprocess.Popen(["rosrun", "package_name", "nodename"])
def f2():
    // NEED CODE HERE 


root.title("root") 
root.geometry("500x500") 
button1 = Button(root,text="CORE", bg="black", fg="white",  width=25, padx = 20, command=f1)
button1.pack()
button2 = Button(root,text="CORE", bg="black", fg="white",  width=25, padx = 20, command=f2)
button2.pack()