[Tutor] The '45' bug in round()

[Kent Johnson]

Dick Moores wrote:

> Kent, I did understand the points you made in that earlier thread. 
> However, I'm unhappy with
> 
>  >>> print round(0.19945,4)
> 0.1994
> 
> Am I the only one unhappy with this kind of rounding?

IMO you are chasing a non-problem. In real-world use, you would probably 
not type in a number and then immediately round it. You would round the 
result of some calculation that itself starts with inexact values and 
then introduces more error. To take the result of this calculation and 
second-guess it seems silly to me.

> My function, round2() restores me to happiness.  :)

Well I should leave well enough alone then :-)

>  >>> print round2(0.19945,4)
> 0.1995
> 
> Is there something wrong with it?

If you want exact decimal representations you may be better off using 
the decimal module than trying to patch binary floating point. Though 
decimal has its own representation error...

Kent