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

[Jeffrey Yasskin]

Jeffrey Yasskin added the comment:

Here's a patch that adds construction from strings (Guido favored them)
and .from_decimal(), changes __init__ to __new__ to enforce
immutability, and removes "rational." from repr and the parens from str.
I don't expect this to be contentious, so I'll commit it tomorrow unless
I hear objections.

Added file: http://bugs.python.org/file9201/rational_tweaks.patch

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