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

[Eric V. Smith]

On 08/31/2016 01:07 PM, MRAB wrote:
> On 2016-08-31 17:19, Guido van Rossum wrote:
>> On Wed, Aug 31, 2016 at 5:21 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:
>>> "h" would be a decent choice - it's not only a continuation of the
>>> e/f/g pattern, it's also very commonly used as a command line flag for
>>> "human-readable output" in system utilities that print numbers.
>>
>> I like it. So after all the drama we're just talking about adding an
>> 'h' format code that's like 'g' but uses SI scale factors instead of
>> exponents. I guess we need to debate what it should do if the value is
>> way out of range of the SI scale system -- what's it going to do when
>> I pass it 1e50? I propose that it should fall back to 'g' style then,
>> but use "engineering" style where exponents are always a multiple of
>> 3.)

Would you also want h to work with integers?

>>> The existing "alternate form" marker in string formatting could be
>>> used to request the use of the base 2 scaling prefixes rather than the
>>> base 10 ones: "#h".
>>
>> Not sure about this one.
>>

'#' already has a meaning for float's 'g' format:

>>> format(1.0, 'g')
'1'
>>> format(1.0, '#g')
'1.00000'

So I think you'd want to pick another type character to mean base 2
scaling, or another character other than #. But it gets cryptic pretty
quickly.

You could indeed use type == 'b' for floats to mean base 2 scaling,
since it has no current meaning, but I'm not sure that's a great idea
because 'b' means binary for integers, and if you want to also be able
to scale ints (see above), then there's a conflict. Maybe type == 'z'?

Or, use something like '@' (or whatever) instead of '#' to mean "the
other alternate form", base 2 scaling.

> Does the 'type' have to be a single character?

As a practical matter, yes, it should just be a single character. You
could make a special case for 'h' and 'hb', but I would not recommend
that. Explaining it in the documentation would be confusing.

Eric.