Significant figures calculation

[Ethan Furman]

Harold Fellermann wrote:
> Hi Ethan,
> 
>>>>>> Empirical('1200.').significance
>>> 2
>>>>>> Empirical('1200.0').significance
>>> 5
>> What about when 1200 is actually 4 significant digits? Or 3?
> 
> Then you'd simply write 1.200e3 and 1.20e3, respectively.
> That's just how the rules are defined.

But your code is not following them:

Python 3.2 (r32:88445, Feb 20 2011, 21:29:02) [MSC v.1500 32 bit 
(Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
--> from decimal import Decimal
--> class Empirical(Decimal) :
...     @property
...     def significance(self) :
...         t = self.as_tuple()
...         if t[2] < 0 :
...             return len(t[1])
...         else :
...             return len(''.join(map(str,t[1])).rstrip('0'))
...
--> Empirical('1.200E+3').significance
2  # should be four
--> Empirical('1.20E+3').significance
2  # should be three
--> Empirical('1.20E+4').significance
2  # should be three

The negatives appear to work, though:
--> Empirical('1.20E-4').significance
3
--> Empirical('1.2819E-3').significance
5
--> Empirical('1.2819E-1').significance
5
--> Empirical('1.281900E-1').significance
7

~Ethan~