Python ROUND_HALF_UP not working as expected

Question

I can\'t seem to understand the decimal docs to get 2.775 to round to 2.78 using the decimal class.

import decimal
decimal.getcontext().prec = 7
print(decima         


        

Answer 1

If you add a single line to your code thus:

print (decimal.Decimal(2.775))

then you'll see why it's rounding down:

2.774999999999999911182158029987476766109466552734375

The value 2.775, as a literal is stored as a double, which has limited precision.

If you instead specify it as a string, the value will be preserved more accurately:

>>> import decimal

>>> print (decimal.Decimal(2.775))
2.774999999999999911182158029987476766109466552734375

>>> print (decimal.Decimal('2.775'))
2.775