Your solution is quite neat. 
But it still misses use cases of the `global` statement:

  1      y = 2
  2
  3      def f():
  4          y = 9
  5  ->     import pdb; pdb.set_trace();
  6
  7      f()
(Pdb) global y; y
9
(Pdb) global y; y += 1; y
10
(Pdb) globals()['y']
2