I've been programming for many years, and recently started learning Python. The following code works as expected in both python 2.5 and 3.0 (on OS X if that matters):
a, b, c = (1, 2, 3)
print(a, b, c)
print(c) # (A)
#c+=1 # (B)
Python treats variables in functions differently depending on whether you assign values to them from within the function or not. If you assign any value to a variable, it is treated by default as a local variable. Therefore, when you uncomment the line, you are attempting to reference a local variable before any value has been assigned to it.
If you want the variable c to refer to the global c put
as the first line of the function.
As of python 3, there is now
that you can use to refer to the nearest enclosing (not necessarily global) scope.