from tkinter import *
a=0
root=Tk()
def timer():
    global a
    a=a+1
    lab['text'] = a
    lab.after(1000, timer)
lab=Label(root, text=a)
lab.pack()
lab.after(1000, timer)
root.mainloop()

from tkinter import *
a=1
root=Tk()
def timer(a):
    a=a+1
    lab['text'] = a
    lab.after(1000, timer(a))
lab=Label(root, text=a)
lab.pack()
lab.after(1000, timer(a))
root.mainloop()

lab.after(1000, lambda: timer(a))

lab.after(1000, lambda: timer(a))

def f():
    return timer(a)
lab.after(1000, f)

lambda: 1

def f():
    return 1

# ...
lab.after(1000, timer, a)
# ...