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

[Greg Ewing]

Random832 wrote:
> One thing to consider is that this is very likely to be used with a unit
> (e.g. "%hA" intending to display in amperes), so maybe it should put a
> space after it? Though really people are probably going to want "1 A" vs
> "1 kA" in that case, rather than "1 A" vs "1kA".

I don't think a space should be automatic. The typographical
recommendation is to put a thin non-breaking space between
the value and the unit, but this is not possible with a
monospaced font, so some people might decide that it's
better without a space, or they might want to use a
character other than 0x20. Better to let the user put the
space in the format string if wanted.

> Engineering or SI-scale-factor format suggests a third
> possibility: number of decimal places to be shown after the displayed
> decimal point, e.g. "%.1h" % 1.2345 * 10 ** x for x in range(10): "1.2",
> "12.3", "123.5", "1.2k", "12.3k", "123.5k", "1.2M", "12.3M", "123.5M".

I'm inclined to think it should be the number of significant
digits, not decimal places, to give a more consistent
precision as the magnitude of the number changes.

For example, if you're displaying some resistor values that
are accurate to 2 digits, you would want to see 2.7k,
27k, 270k, but not 27.0k or 270.0k as those would suggest
spurious precision.

This would also help with fitting the value into a fixed
width, since you would know that a precision of n would
use at most n+1 characters for the numeric part.

-- 
Greg