Calculating float values in Python with builtins.

calculating_floats.py

#!/usr/bin/env python


def p(t, v):
    print('{:<40}-> {}'.format(t, v))

p('.1 + .1 + .1', .1 + .1 + .1)
p('round(.1 + .1 + .1, 10)', round(.1 + .1 + .1, 10))
p('(0.1).as_integer_ratio()', (0.1).as_integer_ratio())


import math

p('sum([0.1] * 10) == 1.0', sum([0.1] * 10) == 1.0)
p('math.fsum([0.1] * 10) == 1.0', math.fsum([0.1] * 10) == 1.0)


from fractions import Fraction
from decimal import Decimal

p('format(0.1, \'.17f\')', format(0.1, '.17f'))
p('format(Decimal.from_float(0.1), \'.17\')', format(Decimal.from_float(0.1), '.17'))
p('Fraction.from_float(0.1)', Fraction.from_float(0.1))
p('Decimal.from_float(0.1)', Decimal.from_float(0.1))

result_python27.txt

.1 + .1 + .1                            -> 0.3
round(.1 + .1 + .1, 10)                 -> 0.3
(0.1).as_integer_ratio()                -> (3602879701896397, 36028797018963968)
sum([0.1] * 10) == 1.0                  -> False
math.fsum([0.1] * 10) == 1.0            -> True
format(0.1, '.17f')                     -> 0.10000000000000001
format(Decimal.from_float(0.1), '.17')  -> 0.10000000000000001
Fraction.from_float(0.1)                -> 3602879701896397/36028797018963968
Decimal.from_float(0.1)                 -> 0.1000000000000000055511151231257827021181583404541015625

result_python3.txt

.1 + .1 + .1                            -> 0.30000000000000004
round(.1 + .1 + .1, 10)                 -> 0.3
(0.1).as_integer_ratio()                -> (3602879701896397, 36028797018963968)
sum([0.1] * 10) == 1.0                  -> False
math.fsum([0.1] * 10) == 1.0            -> True
format(0.1, '.17f')                     -> 0.10000000000000001
format(Decimal.from_float(0.1), '.17')  -> 0.10000000000000001
Fraction.from_float(0.1)                -> 3602879701896397/36028797018963968
Decimal.from_float(0.1)                 -> 0.1000000000000000055511151231257827021181583404541015625