Can you guess why importing the attached x.py does nothing, without
printing "hello" at all?

The real issue shown in that example is that 'return' and 'yield'
outside a function are ignored instead of giving a SyntaxError if they
are optimized away by 'if 0:'.  

(Hint about x.py: the yield sets the CO_GENERATOR flag before it gets
optimized away.  So clearly, that's a way to comment out a whole module
-- just put "if 0: yield" anywhere at top-level...)