Python 2.5 compiles the following piece of code
differently than Python 2.4:

g = 1
def f():
    g += 1

In Python 2.4, this raises an UnboundLocalError. In
current svn trunk, it will increment the global g by 1.
(dis.dis shows that it actually compiles into
LOAD/STORE_GLOBAL opcodes.) It seems the compiler
doesn't treat augmented assignment as assignment for
the purpose of determining locals, as this still fails
correctly:

g = 1
def f():
    g += 1
    g = 5

I can't find where this optimization happens nowadays,
but it feels like a short fix.