Steven D'Aprano added the comment:
As documented, this is not a bug.
"if two multiples are equally close, rounding is done toward the even choice"
https://docs.python.org/3/library/functions.html#round
This is also called "Banker's Rounding" or "Round half to even" and for
rounding many
Spencer Brown added the comment:
This is intentional behaviour, if the value is 5 it rounds towards even as
documented here: https://docs.python.org/3/library/functions.html#round
The reason is so that if you have a bunch of random data, rounding won't
slightly bias the result upward or
New submission from Adam Ulrich :
round(250,-2) returns 200
round(350,-2) returns 400
round(450,-2) returns 400
round(550,-2) returns 600
round(5,-1) returns 0
round(15,-1) returns 20
round(500,-3) returns 0
round(1500,-3) returns 2000
expected: round of 5 to consistently rounds up.
