[issue1682] Move Demo/classes/Rat.py to Lib/rational.py and fix it up.

[Jeffrey Yasskin]

Jeffrey Yasskin added the comment:

Thanks! I've added some minimal documentation and construction from
other Rationals. The other formats for string parsing center around
where whitespace is allowed, and whether you can put parens around the
whole fraction. Parens would, of course, go away if str() no longer
produces them. So they're not significantly different. I guess my
objection to construction from strings is mostly that I'm ambivalent,
and especially for a core library, that means no. If there's support
from another core developer or two, I'd have no objections.

Inexact is saying that one thing could be ==3 and the other ==0, so I
think it's correct.

Negative zeros are a problem. :) I think the default implementations are
still fine, but you're right that classes like Decimal will need to
think about it, and the numbers module should mention the issue. It's
not related to the Rational implementation though, so in another patch.

I've submitted this round as r59974. Onto the next patch! :)

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