I recently had problems with this, too. Hence, I have developed a python 3 module that has 2 functions trueround() and trueround_precision() that address this and give the same rounding behaviour we are used to from primary school (not banker's rounding). Here is the module. Just save the code and copy it in or import it.
Note: the trueround_precision module can change the rounding behaviour depending on needs according to the ROUND_CEILING, ROUND_DOWN, ROUND_FLOOR, ROUND_HALF_DOWN, ROUND_HALF_EVEN, ROUND_HALF_UP, ROUND_UP, and ROUND_05UP flags in the decimal module (see that modules documentation for more info). For the functions below, see the docstrings or use help(trueround) and help(trueround_precision) if copied into an interpreter for further documentation.
#! /usr/bin/env python3
# -*- coding: utf-8 -*-
def trueround(number, places=0):
'''
trueround(number, places)
example:
>>> trueround(2.55, 1) == 2.6
True
uses standard functions with no import to give "normal" behavior to
rounding so that trueround(2.5) == 3, trueround(3.5) == 4,
trueround(4.5) == 5, etc. Use with caution, however. This still has
the same problem with floating point math. The return object will
be type int if places=0 or a float if places=>1.
number is the floating point number needed rounding
places is the number of decimal places to round to with '0' as the
default which will actually return our interger. Otherwise, a
floating point will be returned to the given decimal place.
Note: Use trueround_precision() if true precision with
floats is needed
GPL 2.0
copywrite by Narnie Harshoe <[email protected]>
'''
place = 10**(places)
rounded = (int(number*place + 0.5if number>=0 else -0.5))/place
if rounded == int(rounded):
rounded = int(rounded)
return rounded
def trueround_precision(number, places=0, rounding=None):
'''
trueround_precision(number, places, rounding=ROUND_HALF_UP)
Uses true precision for floating numbers using the 'decimal' module in
python and assumes the module has already been imported before calling
this function. The return object is of type Decimal.
All rounding options are available from the decimal module including
ROUND_CEILING, ROUND_DOWN, ROUND_FLOOR, ROUND_HALF_DOWN, ROUND_HALF_EVEN,
ROUND_HALF_UP, ROUND_UP, and ROUND_05UP.
examples:
>>> trueround(2.5, 0) == Decimal('3')
True
>>> trueround(2.5, 0, ROUND_DOWN) == Decimal('2')
True
number is a floating point number or a string type containing a number on
on which to be acted.
places is the number of decimal places to round to with '0' as the default.
Note: if type float is passed as the first argument to the function, it
will first be converted to a str type for correct rounding.
GPL 2.0
copywrite by Narnie Harshoe <[email protected]>
'''
from decimal import Decimal as dec
from decimal import ROUND_HALF_UP
from decimal import ROUND_CEILING
from decimal import ROUND_DOWN
from decimal import ROUND_FLOOR
from decimal import ROUND_HALF_DOWN
from decimal import ROUND_HALF_EVEN
from decimal import ROUND_UP
from decimal import ROUND_05UP
if type(number) == type(float()):
number = str(number)
if rounding == None:
rounding = ROUND_HALF_UP
place = '1.'
for i in range(places):
place = ''.join([place, '0'])
return dec(number).quantize(dec(place), rounding=rounding)
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