[Tutor] learning nested functions

[Tim Hanson]

In  the first Lutz book, I am learning about nested functions.

Here's the book's example demonstrating global scope:
>>> def f1():
	x=88
	def f2():
		print(x)
	f2()

	
>>> f1()
88

No problem so far.  I made a change and ran it again:

>>> def f1():
	x=88
	def f2():
		print(x)
		x=99
		print(x)
	f2()

	
>>> f1()
Traceback (most recent call last):
  File "<pyshell#11>", line 1, in <module>
    f1()
  File "<pyshell#10>", line 7, in f1
    f2()
  File "<pyshell#10>", line 4, in f2
    print(x)
UnboundLocalError: local variable 'x' referenced before assignment

This doesn't work.  To my mind,in f2() I first print(x) then assign a variable 
with the same name in the local scope, then print the changed x.  Why doesn't 
this work?