[Python-Dev] Re: [Python-checkins] CVS: python/dist/src/Doc/tut tut.tex,1.133.2.1,1.133.2.2

[Guido van Rossum]

> Actually, 31.400000000000002 wasn't a true improvement over the earlier 31.4:
> so long as we rely on the platform C to format floats, the output isn't
> well-defined (the last digit or so can and will vary across boxes).

I can't check right now, but I thought that this was pretty consistent
across some common platforms?

> I can certainly explain that this is so, and even why, but unsure
> the tutorial is the right place for it.  In any case the tutorial
> shouldn't be giving examples whose output is platform-dependent.
> For example, don't use 10 * 3.14, use 10 * 3.25.  Want me to scour
> the tutorial for all such cases?

Are you serious?

This is something that the newbie wou is in the least bit adventurous
will run into anyway, so I don't think that not talking about this at
all in the tutorial is fair or helpful.  That just perpetuates the
questions from newbies about "floating point is broken" -- since none
of the tutorial examples prepare them for this.

Since this is behavior that is ordinarily observed and perpetually
perplexing, I think it *must* be treated in the tutorial.  The
tutorial doesn't have to have the full explanation -- maybe it's
enough to say something like ``due to round-off errors you will
sometimes see inexact results like 31.400000000000002; don't worry
about this, you can use str() or "%g" (but not round()!) to strip
redundant precision, and here's a URL for more info.''

Or maybe the full story can be an appendix.

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