python incorrect rounding with floating point numbers -

>>> = 0.3135 >>> print("%.3f" % a) 0.314 >>> = 0.3125 >>> print("%.3f" % a) 0.312 >>> 

i expecting 0.313 instead of 0.312 thought on why this, , there alternative way can use 0.313?

thanks

python 3 rounds according ieee 754 standard, using round-to-even approach.

if want round in different way implement hand:

import math def my_round(n, ndigits):     part = n * 10 ** ndigits     delta = part - int(part)     # round "away 0"     if delta >= 0.5 or -0.5 < delta <= 0:         part = math.ceil(part)     else:         part = math.floor(part)     return part / (10 ** ndigits) 

example usage:

in [12]: my_round(0.3125, 3) out[12]: 0.313 

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note: in python2 rounding away zero, while in python3 rounds even. (see, example, difference in documentation round function between 2.7 , 3.3).