Python arithmetic with small numbers

Question

I am getting the following unexpected result when I do arithmetic with small numbers in Python:

>>> sys.float_info
sys.float_info(max=1.797693134862         


        

Answer 1

If you need this level of precision, consider the Decimal module

>>> decimal.Decimal(1.0)-decimal.Decimal('1.0e-17')
Decimal('0.999999999999999990')
>>> decimal.Decimal(1.0)-decimal.Decimal('1.0e-17')

And:

>>> decimal.Decimal(1.0)-decimal.Decimal('1.0e-17')<1.0
True

Careful with the last one though because you can get conversion errors.

Others have suggested What Every Computer Scientist Should Know About Floating-Point Arithmetic. and I also recommend Don’t Store That in a Float