[issue4796] Decimal to receive from_float method

[Mark Dickinson]

Mark Dickinson <dickinsm at gmail.com> added the comment:

One also has to worry about the exponent of the converted result:  e.g., 
should Decimal.from_float(10.0) produce Decimal('1E1') or Decimal('10')?
The latter looks nicer, to me.

IEEE 754 isn't much help here:  as far as I can tell it says nothing about 
binary <-> decimal conversions.

I see two reasonable strategies:  (1) always use the largest exponent 
possible (so we'd get Decimal('1E1') above), or (2) when the quantity 
converted is an exact integer, use an exponent of zero; otherwise fall 
back to (1).

Option (2) is pretty much what the recipe in the docs does already, I 
think:  it computes a quotient of two Decimals, each having exponent zero, 
so the preferred exponent of the result is also zero.

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