Round Function Issue

Bug report

Bug description:

When using the round function, there is an issue when I try to round the floating-point number that has a even number to the left of the decimal, and 5 to right of the decimal. For example 2.5, 4.5, 6.5 etc... Instead of rounding the number up , it rounds the number down. Example: Instead of rounding 2.5 up to 3, it rounds it down to 2. As I mentioned, this only happens to the even number. For odd numbers, it actually rounds the odd number up. Example: 3.5 will be rounded up to 4

print(round(2.5))
print(round(3.5))
print(round(4.5))

CPython versions tested on:

3.13

Operating systems tested on:

Windows

[hugovk]

Duplicate of #125155:

This is documented behavior:

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)

See also this SO question.

[efimov-mikhail]

It's not a bug, round function is designed this way:

"For the built-in types supporting round(), values are rounded to the closest multiple of 10 to the power minus ndigits; 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)."

Thanks to those who commented on this. It seems weird how the function was programmed to round that way, but I understand it now. Thanks again.

[skirpichev]

It's not "weird", see https://en.wikipedia.org/wiki/Rounding#Rounding_half_to_even

This is the default rounding mode used in IEEE 754.