How to round float 0.5 up to 1.0, while still rounding 0.45 to 0.0, as the usual school rounding?

Question

It appears that the default Python round(1 / 2) gives 0.

How to round float 0.5 up to 1.0, while still rounding 0.45 to 0.0, as the usual school rounding?

Answer 1

It is actually currently considered proper to not blindly round *.5 up. Rather, it is proper to round *.5 to the nearest even number. Python 3 implements this "proper" form of "banker rounding", but a lot of other languages don't (yet). Blindly rounding *.5 up produces a slight bias, but "banker rounding" helps to balance it it out. See this thread for more info. So...

Method 1

You could conditionally use a ceil(...) function (from the math module) for the rounding up aspect. You'll have to do it conditionally in order to also maintain the regular rounding behavior for values less than 0.5. Try something like the following (note that this isn't extremely robust in that it only works on positive values...it should be able to be easily adapted to work with both positive and negative values though):

import math

val = 1.5
x = 0

if (float(val) % 1) >= 0.5:
    x = math.ceil(val)
else:
    x = round(val)

Note that a ceil(...) function will return an integer, not a float. This shouldn't be a major issue, but now you are aware.

Method 2

From the post I linked to above, it looks like another option is to use the decimal module to emulate the "old" way of rounding's behavior. I'm kind of copy & pasting from there, but here you go:

import decimal

x = decimal.Decimal('1.5').quantize(decimal.Decimal('1'), 
rounding=decimal.ROUND_HALF_UP)

Supposedly the decimal.ROUND_HALF_UP form of rounding is what you are looking for. This way you don't have to use a ceil(...) function conditionally.