Why Does Float Rounding Surprise with Even Results?

Half Float Rounding-Up Dilemma

Encountering an oddity with the round() function? Observe the following behavior:

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

You might expect the floating values to consistently round up, yet they surprisingly round to the nearest even number.

Reason Behind the Behavior

The documentation for Numeric Types clarifies this peculiar behavior with the statement "round(x[, n]) x rounded to n digits, rounding half to even." This is known as bankers rounding. Instead of persistent rounding up or down, which would amplify rounding errors, bankers rounding compensates by targeting the closest even number.

Solution for Controlled Rounding

To handle rounding precisely, leverage the decimal module. This module equips you with options to specify specific rounding strategies. For instance, 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

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