python - Unexplained Infinite Loop in Tkinter -

i learning tkinter python2 came across code.this creates window in tkinter , increments value of label every 1 second while code runs fine..can tell me why infinite loop not observed after every thousand seconds control return count() , control should have never reached last line of code?

import tkinter tk  counter = 0  def counter_label(label):   def count():     global counter     counter += 1     label.config(text=str(counter))     label.after(1000, count)   count()   root = tk.tk() root.title("counting seconds") label = tk.label(root, fg="green") label.pack() counter_label(label) button = tk.button(root, text='stop', width=25, command=root.destroy) button.pack() root.mainloop() 

also label variable passed function def counter_label copy of original variable changing shouldnot affect original variable.how happening?

any appreciated.

1. 1000 measured in milliseconds i.e., 1 second
2. count() reached , infinite loop (if root.mainloop() running).

label.after(1000, count) schedules count function , returns immediately. tkinter event loop should running otherwise count() won't called again.