global variable not working inside function. Increment

[Helvin]

Hi,

This increment thing is driving me nearly to the nuts-stage. > <

I have a function that allows me to pick points. I want to count the
number of times I have picked points.

    global no_picked
    no_picked = 0

    def picked(object, event):
          no_picked += 1
          print no_picked

Error msg says: UnboundLocalError: local variable 'no_picked'
referenced before assignment
For some reason, no_picked does not increment, but the printing
statement works.

Do you know why?

(I'm actually writing this for a vtkrenderwindowinteractor.)

Helvin