问题

The behavior of round() for floats can be surprising: for example, round(2.675, 2) gives 2.67 instead of the expected 2.68. This is not a bug: it’s a result of the fact that most decimal fractions can’t be represented exactly as a float. See Floating Point Arithmetic: Issues and Limitations for more information.

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使用python3实现真四舍五入round()函数代码

#! /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

'''

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

'''

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)