[Python-ideas] SI scale factors in Python

[Ken Kundert]

> Question, though: What happens with exa-? Currently, if the parser
> sees "1E", it'll expect to see another number, eg 1E+1 == 10.0. Will
> this double meaning cause confusion?

Allow me to refine my answer to this question ...

Yes, that is definitely problematic. I see two possible solutions.
1. Limit ourselves to the common scale factors:
     T, G, M, k, _, m, u, n, p, f, a
2. Or accept X in lieu of E. After all, the e is silent anyway.  Thus, on input 
   we accept ...

    1Y -> 1e+24
    1Z -> 1e+21
 -> 1X -> 1e+18 <- only difference
    1P -> 1e+15
    1T -> 1e+12
    1G -> 1e+09
    1M -> 1e+06
    1k -> 1e+03
    1_ -> 1e+00
    1m -> 1e-03
    1u -> 1e-06
    1n -> 1e-09
    1p -> 1e-12
    1f -> 1e-15
    1a -> 1e-18
    1z -> 1e-21
    1y -> 1e-24

But on output we use ...

    1Y -> 1e+24  optional
    1Z -> 1e+21  optional
 -> 1E -> 1e+18  optional
    1P -> 1e+15  optional
    1T -> 1e+12
    1G -> 1e+09
    1M -> 1e+06
    1k -> 1e+03
    1_ -> 1e+00
    1m -> 1e-03
    1u -> 1e-06
    1n -> 1e-09
    1p -> 1e-12
    1f -> 1e-15
    1a -> 1e-18
    1z -> 1e-21  optional
    1y -> 1e-24  optional

The optional scale factors are unfamiliar to most people, and if used might 
result in harder to read numbers. So I propose that '%r' only outputs the common 
scale factors, and %R outputs all the scale factors. Or we can use '#' in the 
format string to indicate the 'alternate' form should be used, in this case 
'alternate' means that the extended set of scale factors should be used.

-Ken