function inside a function - global and nonlocal scope

Question

I'm trying out the following code with global, and nonlocal scope. following snippet working wihtout any issues.

def countdown(start):
  n = start
  def display():
    print('--> %d' % n)
  def decrement():
    nonlocal n ##using python3
    n -= 1
  while n > 0:
    display()
    decrement()
countdown(10)

countdown(10)

but why can't i use the global n ? instead of nonlocal n. that gives me

 UnboundLocalError: local variable 'n' referenced before assignment

this is the snippet

 def countdown(start):
   global n   ##defined it global
   n = start
   def display():
     print('--> %d' % n)
   def decrement():
     ##no nonlocal varibale here
     n -= 1
   while n > 0:
     display()
     decrement()

countdown(10)

Answer 1

The global declaration doesn't automatically apply to nested functions. You need another declaration:

def decrement():
    global n
    n -= 1

so n in decrement also refers to the global variable.

Answer 2

You need to mark a variable as global in every function where you use it (or rather, every one where you assign to it). You marked n as global in countdown, but decrement still thinks it is local. If you want decrement to also use the global n, you need to put another global n inside decrement.