[Python-ideas] real numbers with SI scale factors: next steps

[Steven D'Aprano]

On Tue, Aug 30, 2016 at 01:34:27PM -0700, Ken Kundert wrote:

> 3. A change to the various string formatting mechanisms to allow outputting real 
>    numbers with SI scale factors:

This is somewhat similar to a library I wrote for formatting bytes:

https://pypi.python.org/pypi/byteformat

Given that feature freeze for 3.6 is two weeks way, I don't think that 
this proposal will appear before 3.7. So I'm interested, but I'm less 
interested *right now*. So for now I'll limit myself to only a few 
observations.

 
>       >>> print('Speed of light in a vacuum: {:r}m/s.'.format(2.9979e+08))
>       Speed of light in a vacuum: 299.79 Mm/s.

Do you think that {:r} might be confused with {!r}?

What's the mnemonic here? Why "r" for scale factor?

 
>       >>> print('Speed of sound in water: %rm/s.' % 1481
>       Speed of sound in water: 1.481 km/s.

I doubt that you'll get any new % string formatting codes. That's a 
legacy interface, *not* deprecated but unlikely to get new features 
added, and it is intended to closely match the C printf codes.


A few more questions:

(1) Why no support for choosing a particular scale? If this only 
auto-scales, I'm not interested.

(2) Support for full prefix names, so we can format (say) "kilograms" as 
well as "kg"?

(3) Scientific notation and engineering notation?

(4) 1e5 versus 1×10^5 notation?

(5) Is this really something that format() needs to understand? We can 
get a *much* richer and more powerful interface by turning it into a 
generalise numeric pretty-printing library, at the cost of a little less 
convenience.


> 3. Allowing numbers to be formatted with SI prefixes is useful and not 
>    controversial.

I wouldn't quite go that far. You made an extremely controversial 
request (new syntax for scaling prefixes + ignored units) and nearly all 
the attention was on that.

For what its worth, I have no need for a format code which *only* 
auto-selects the scaling factor. If I don't have at least the option to 
choose which scaling factor I get, and hence the prefix, this is of 
little or no use to me, I likely wouldn't use it, and as far as I am 
concerned the nuisance value of having yet another format string code to 
learn outweighs the benefit.



-- 
Steve