>>> a=666       # global
>>> def fun1():
...    print a  # global
... 
>>> def fun2():
...    print a  # privat
...    a = 33   # declaration
... 
>>> fun1()
666
>>> fun2()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in fun2
UnboundLocalError: local variable 'a' referenced before assignment
>>>