Steven D'Aprano <[email protected]> added the comment:
If you change the starting point of the rounding away from zero, the bias flips
back and forth, which is exactly what I would expect from Banker's Rounding:
def check_bias(start):
d = 0.001
ne = no = 0
for i in range(1000):
digit = int(round(start + i * d, 1) * 10)
if digit & 1:
no += 1
else:
ne += 1
return ne, no
# Python 3.7
>>> check_bias(0.0)
(501, 499)
>>> check_bias(0.1)
(500, 500)
>>> check_bias(0.2)
(499, 501)
>>> check_bias(0.3)
(499, 501)
>>> check_bias(0.4)
(500, 500)
>>> check_bias(0.5)
(499, 501)
>>> check_bias(0.6)
(501, 499)
I ran the same check_bias in Python 2.7, which doesn't use bankers rounding,
and the bias is consistently in one direction:
# Python 2.7
>>> check_bias(0.0)
(500, 500)
>>> check_bias(0.1)
(499, 501)
>>> check_bias(0.2)
(498, 502)
>>> check_bias(0.3)
(498, 502)
>>> check_bias(0.4)
(499, 501)
>>> check_bias(0.5)
(498, 502)
>>> check_bias(0.6)
(500, 500)
----------
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Python tracker <[email protected]>
<https://bugs.python.org/issue41198>
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