[Python-Dev] Round Bug in Python 1.6?

[Guido van Rossum]

> asa side effect, I happened to observe the following rounding bug.
> It happens in Stackless Python, which is built against the
> pre-unicode CVS branch.
> 
> Is this changed for 1.6, or might it be my bug?
> 
> D:\python\spc>python
> Python 1.5.42+ (#0, Mar 29 2000, 20:23:26) [MSC 32 bit (Intel)] on win32
> Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
> >>> round(3.1415926585, 4)
> 3.1415999999999999
> >>> ^Z
> 
> D:\python>python
> Python 1.5.2 (#0, Apr 13 1999, 10:51:12) [MSC 32 bit (Intel)] on win32
> Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
> >>> round(3.1415926585, 4)
> 3.1416
> >>> ^Z

This is because repr() now uses full precision for floating point
numbers.  round() does what it can, but 3.1416 just can't be
represented exactly, and "%.17g" gives 3.1415999999999999.

This is definitely the right thing to do for repr() -- ask Tim.

However, it may be time to switch so that "immediate expression"
values are printed as str() instead of as repr()...

--Guido van Rossum (home page: http://www.python.org/~guido/)