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This is a bit tricky. The first expression you suggest doesn't work at all for negative numbers (e.g., producing `-3.0` for `round(-1.8)`).
The second expression *mostly* works as you want, but not entirely. Some examples:
>>> def f(n): return float(math.trunc(n + math.copysign(0.5, n)))
... 
>>> x = 0.5 - 2**-54
>>> x
0.49999999999999994
>>> f(x) # should be 0.0
1.0
>>> x = 5e15 + 3.0
>>> x
5000000000000003.0
>>> f(x) # should be x again
5000000000000004.0
And neither of these addresses the two-argument case of `round`.
It may be better to simply flag this as something that 2to3 can't handle, and that needs a manual check.