stopwatch.py

import tkinter as Tkinter
from datetime import datetime
counter = 0
running = False


def counter_label(label):
    def count():
        if running:
            global counter
			# To manage the intial delay. 
            if counter == 0:
                display = 'Ready!'
            else:
                tt = datetime.utcfromtimestamp(counter)
                string = tt.strftime('%H:%M:%S')
                display = string
	
            label['text'] = display
	
			# label.after(arg1, arg2) delays by 
			# first argument given in milliseconds 
			# and then calls the function given as second argument. 
			# Generally like here we need to call the 
			# function in which it is present repeatedly. 
			# Delays by 1000ms=1 seconds and call count again. 
            label.after(1000, count)
            counter += 1
	
	# Triggering the start of the counter. 
    count()
	

# start function of the stopwatch 
def Start(label):
    global running
    running = True
    counter_label(label)
    start['state'] = 'disabled'
    stop['state'] = 'normal'
    reset['state'] = 'normal'
	

# Stop function of the stopwatch 
def Stop():
    global running
    start['state'] = 'normal'
    stop['state'] = 'disabled'
    reset['state'] = 'normal'
    running = False
	

# Reset function of the stopwatch 
def Reset(label):
	global counter
	counter = 0
	# If reset is pressed after pressing stop. 
	if not running:
		reset['state'] = 'disabled'
		label['text'] = '00:00:00'
	# If reset is pressed while the stopwatch is running. 
	else:
		label['text'] = '00:00:00'


root = Tkinter.Tk()
root.title("Stopwatch")

# Fixing the window size.
root.minsize(width=250, height=70)
label = Tkinter.Label(root, text='Ready!', fg='black', font='Verdana 30 bold')
label.pack()
f = Tkinter.Frame(root)
start = Tkinter.Button(f, text='Start', width=6, command=lambda: Start(label))
stop = Tkinter.Button(f, text='Stop', width=6, state='disabled', command=Stop)
reset = Tkinter.Button(f, text='Reset', width=6, state='disabled', command=lambda: Reset(label))
f.pack(anchor='center', pady=5)
start.pack(side='left')
stop.pack(side='left')
reset.pack(side='left')
root.mainloop()