How to properly round up half float numbers in Python?

Question

I am facing a strange behavior of the round() function:

for i in range(1, 15, 2):
    n = i / 2
    print(n, "=>", round(n))

This code prints:

0.5 => 0
1.5 => 2
2.5 => 2
3.5 => 4
4.5 => 4
5.5 => 6
6.5 => 6

I expected the floating values to be always rounded up, but instead, it is rounded to the nearest even number.

Why such behavior, and what is the best way to get the correct result?

I tried to use the fractions but the result is the same.

Answer 1

The Numeric Types section documents this behaviour explicitly:

round(x[, n])
x rounded to n digits, rounding half to even. If n is omitted, it defaults to 0.

Note the rounding half to even. This is also called bankers rounding; instead of always rounding up or down (compounding rounding errors), by rounding to the nearest even number you average out rounding errors.

If you need more control over the rounding behaviour, use the decimal module, which lets you specify exactly what rounding strategy should be used.

For example, to round up from half:

>>> from decimal import localcontext, Decimal, ROUND_HALF_UP
>>> with localcontext() as ctx:
...     ctx.rounding = ROUND_HALF_UP
...     for i in range(1, 15, 2):
...         n = Decimal(i) / 2
...         print(n, '=>', n.to_integral_value())
...
0.5 => 1
1.5 => 2
2.5 => 3
3.5 => 4
4.5 => 5
5.5 => 6
6.5 => 7