Python optimisation has surprising side effects

Here's something that surprised me:

a = None
def f():
    b = a
    return b
def g():
    b = a
    a = 'foo'
    return b

While f() is perfectly fine, g() raises an UnboundLocalError. This is because Python optimises access to local variables using the LOAD_FAST/STORE_FAST opcode, you can easily see why this is looking at the code objects of those functions:

>>> f.__code__ .co_names
>>> f.__code__ .co_varnames
('a', 'b')
>>> g.__code__ .co_names
>>> g.__code__ .co_varnames

I actually found out this difference thanks to finally watching the Optimizations And Micro-Optimizations In CPython talk by Larry Hastings from PyCon 2010. I never realised that you could create a situation where the nonlocal scope would not be looked in.