[Python-ideas] real numbers with SI scale factors

[Steven D'Aprano]

On Mon, Aug 29, 2016 at 08:48:55PM -0700, Ken Kundert wrote:

>     >>> print(d_sun.to(u.kpc))
>     4.850441695494146e-09 kpc
> 
> I can see where this can be helpful at times, but it kind of goes against the 
> spirit of SI scale factors, were you are generally expected to 'normalize' the 
> scale factor (use the scale factor that results in the digits presented before 
> the decimal point falling between 1 and 999). So I would expected
> 
>     d_andromeda = 780 kpc
>     d_sun = 4.8504 upc

Let me see if I get this straight... you *explicitly* asked for the 
distance to the sun in kpc (kiloparsecs), but you expected a result in 
µpc (microparsecs)?

When you ask the waiter for a short black, do you get upset that 
he doesn't bring you a latte with soy milk? *wink*

I can see that such a normalising function would be useful, but I don't 
think it should be the default. (If I ask for millimetres, I want 
millimetres, not gigametres.) I've written and used code like that for 
bytes, it makes sense to apply it to other measurement units. But only 
if the caller requests normalisation, never by default.

I don't think there is any such general expectation that values should 
be normalised in that way, and certainly not that your conversion 
program should automatically do it for you. For example, see this list 
of long-lived radioactive isotopes:

http://w.astro.berkeley.edu/~dperley/areopagus/isotopetable.html

Values above 650,000,000,000 (650e9) years are shown in "scientific 
format", not "engineering format", e.g. Selenium-82 is given 
as 1.1 x 10^20 rather than 110 x 10^18.

Likewise:

http://www.nist.gov/pml/data/halflife-html.cfm

displays a range of units (minutes, hours, days) with the base value 
ranging up to over ten thousand, e.g. Ti-44 is shown as 22154 ± 456 
days. This is NIST, which makes it pretty official. I don't think 
there's any general expectation that values should be shown in the range 
1 to 999. (Perhaps in certain specialist areas.)



-- 
Steve