Exploring the cost of defining a noop function inline, vs. calling a free noop function:
In [215]: %timeit test_inline()
1000000 loops, best of 3: 261 ns per loop
In [216]: %timeit test_external()
1000000 loops, best of 3: 215 ns per loop
In [217]: %timeit y.test_inline()
1000000 loops, best of 3: 306 ns per loop
In [218]: %timeit y.test_external()
1000000 loops, best of 3: 267 ns per loop