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

[Guido van Rossum]

On Wed, Aug 31, 2016 at 5:21 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:
> On 31 August 2016 at 17:07, Chris Angelico <rosuav at gmail.com> wrote:
>> On Wed, Aug 31, 2016 at 2:08 PM, Ken Kundert
>> <python-ideas at shalmirane.com> wrote:
>>> > What's the mnemonic here? Why "r" for scale factor?
>>>
>>> My thinking was that r stands for real like f stands for float.
>>> With the base 2 scale factors, b stands for binary.
>>
>> "Real" has historically often been a synonym for "float", and it
>> doesn't really say that it'll be shown in engineering notation. But
>> then, we currently have format codes 'e', 'f', and 'g', and I don't
>> think there's much logic there beyond "exponential", "floating-point",
>> and... "general format"? I think that's a back-formation, frankly, and
>> 'g' was used simply because it comes nicely after 'e' and 'f'. (C's
>> decision, not Python's, fwiw.) I'll stick with 'r' for now, but it
>> could just as easily become 'h' to avoid confusion with %r for repr.
>
> "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.)

> 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.

-- 
--Guido van Rossum (python.org/~guido)