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18 Oct 2021

211018-1510 Python rounding behaviour

round() has weirdly unexpected behaviour that I’m ashamed I didn’t notice or know about:

if two multiples are equally close, rounding is done toward the even choice (so, for example, both round(0.5) and round(-0.5) are 0, and round(1.5) is 2) 1

So:

>>> round(1.5)
2
>>> round(2.5)
2
>>> round(3.5)
4

  1. Built-in Functions — Python 3.10.0 documentation ↩︎

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