Rounding to two decimal places in Python 2.7?

Question

Using Python 2.7 how do I round my numbers to two decimal places rather than the 10 or so it gives?

print \"financial return of outcome 1 =\",\"$\"+str(out1)         


        

Answer 1

Since you're talking about financial figures, you DO NOT WANT to use floating-point arithmetic. You're better off using Decimal.

>>> from decimal import Decimal
>>> Decimal("33.505")
Decimal('33.505')

Text output formatting with new-style format() (defaults to half-even rounding):

>>> print("financial return of outcome 1 = {:.2f}".format(Decimal("33.505")))
financial return of outcome 1 = 33.50
>>> print("financial return of outcome 1 = {:.2f}".format(Decimal("33.515")))
financial return of outcome 1 = 33.52

See the differences in rounding due to floating-point imprecision:

>>> round(33.505, 2)
33.51
>>> round(Decimal("33.505"), 2)  # This converts back to float (wrong)
33.51
>>> Decimal(33.505)  # Don't init Decimal from floating-point
Decimal('33.50500000000000255795384873636066913604736328125')

Proper way to round financial values:

>>> Decimal("33.505").quantize(Decimal("0.01"))  # Half-even rounding by default
Decimal('33.50')

It is also common to have other types of rounding in different transactions:

>>> import decimal
>>> Decimal("33.505").quantize(Decimal("0.01"), decimal.ROUND_HALF_DOWN)
Decimal('33.50')
>>> Decimal("33.505").quantize(Decimal("0.01"), decimal.ROUND_HALF_UP)
Decimal('33.51')

Remember that if you're simulating return outcome, you possibly will have to round at each interest period, since you can't pay/receive cent fractions, nor receive interest over cent fractions. For simulations it's pretty common to just use floating-point due to inherent uncertainties, but if doing so, always remember that the error is there. As such, even fixed-interest investments might differ a bit in returns because of this.