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I'm -0 on this proposal.
There is a small speed-up to be had here:
 $ python3.4 -m timeit -s 'f=lambda kv: kv[0]' -s 's=[10, 20]' 'f(s)'
 10000000 loops, best of 3: 0.105 usec per loop
 $ python3.4 -m timeit -s 'import operator' -s 'f=operator.itemgetter(0)' -s 's=[10, 20]' 'f(s)'
 10000000 loops, best of 3: 0.0717 usec per loop
That said, it is applied only n-times and is likely insignificant when compared to the O(n log n) sort. I think it is better to not add an intermodule dependency when we don't have to.
Also if you're using CPython, then the C version of JSON is used instead of this code. So, any timings or optimizations should be tested against the other implementations (Jython or PyPy) that use them. In those implementations, I don't think itemgetter() has any performance benefit over the lambda.