You can use Decimal built-in also:

  In [1]: from decimal import Decimal as D
  In [2]: D('9999999999999999.0') - D('9999999999999998.0')
  Out[2]: Decimal('1.0')

From docs: Decimal “is based on a floating-point model which was designed with people in mind, and necessarily has a paramount guiding principle – computers must provide an arithmetic that works in the same way as the arithmetic that people learn at school.”

https://docs.python.org/3.6/library/decimal.html