I cannot get the restart button to work on my stopwatch. Using Pythonista on iPhone

import ui
from time import *
start = int(time())
def stop_time(sender):
    finish = int(time())
    total_time = int(finish - start)
    button1 = str("Your time is %i seconds." % (total_time))
    sender.title = None
    sender.title = str(button1)

How do I allow the restart button to change the start variable?

def restart_time(sender):
    start = int(time())
    button2 = str("Stopwatch restarted.")
    sender.title = None
    sender.title = str(button2)
ui.load_view('stop_time').present('sheet')