python incorrect rounding with floating point numbers

Question

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

I am expecting 0.313 instead of 0.312 Any thought on why is this, and is there alternative way I can use to get 0.313?

Thanks

Answer 1

Python 3 rounds according to the IEEE 754 standard, using a round-to-even approach.

If you want to round in a different way then simply implement it by hand:

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

Example usage:

In [12]: my_round(0.3125, 3)
Out[12]: 0.313

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Note: in python2 rounding is always away from zero, while in python3 it rounds to even. (see, for example, the difference in the documentation for the round function between 2.7 and 3.3).