Newbie question

[Ante]

from file 'dummy.py'
----------------------------
num = 0

def dummy():
    num += 1

dummy()
----------------------------

now, in the interpreter:

>>> import dummy
Traceback (most recent call last):
  File "<string>", line 1, in ?
  File "C:\downloads\python2.2.1\dummy.py", line 6, in ?
    dummy()
  File "C:\downloads\python2.2.1\dummy.py", line 4, in dummy
    num += 1
UnboundLocalError: local variable 'num' referenced before assignment


Why does this happen?
If I replace num += 1 with print num, it prints 0 without an error.
I guess it scans the whole function and check if there's an assignment to a
name,
and if there is, assumes it's a local variable. But += isn't supposed to
move the
reference but instead change the same variable. Or what did I get wrong?

Ante.