[issue4796] Decimal to receive from_float method

[Raymond Hettinger]

Raymond Hettinger <rhettinger at users.sourceforge.net> added the comment:

> once you've converted your float to Decimal it's trivial 
> to round it to whatever precision you feel like, so #2 
> seems unnecessary to me.

Agree with Mark on how to control rounding.  The DecimalWay(tm) is that
used by Context.create_decimal().  A second argument is problematic for
two reasons.  First, it demands that some rounding take place but
doesn't let you control the rounding method or signal an Inexact
conversion -- you need a context for that.  Second, the API for decimal
is already somewhat complex and we don't want to make it worse by
introducing a new variant with a second argument that has no parallel
elsewhere in the API.

Also agree with Mark regarding Decimal.from_float() which needs to be
lossless and exact.  It parallels fractions.from_float() which does the
same thing for rationals.

The DecimalWay(tm) is to have Decimal constructors be exact and to use
Context based constructors when rounding is desired.

I'll submit a patch to this effect (unless someone beats me to it).

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue4796>
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