«leftbraned

I discovered this subtle change when one of my unit tests mysteriously failed once I ported it from Python 2 to Python 3. A computed price was consistently one microdollar higher in the Python 3 version.

I eventually tracked it down to a change in rounding behaviour between Python 2 and 3. According to the docs (emphasis mine):

if two multiples are equally close, rounding is done toward the even choice

I proved it to myself thusly:

$ python2 -c 'print( round(8.5) )'
9.0
$ python3 -c 'print( round(8.5) )'
8

This method of rounding is called “banker’s rounding.” The good news is that banker’s rounding better suits our need for a fair rounding method. That is: our goal in rounding is to have rounding errors even out in aggregate, and banker’s rounding achieves that better than always rounding up (i.e., away from zero).

Revert to Python 2 Behaviour

Should you find the need to use Python 2’s rounding algorithm, it’s available as part of the Decimal module as decimal.ROUND_HALF_UP (example).

References

• Python 3.x rounding behavior
• The key differences between Python 2.7.x and Python 3.x with examples