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 exp

Answer 1

If you need accuracy don't use float, use Decimal

>>> from decimal import *
>>> d = Decimal(0.3125)
>>> getcontext().rounding = ROUND_UP
>>> round(d, 3)
Decimal('0.313')

or even Fraction