[Python-Dev] PEP 515: Underscores in Numeric Literals

[Petr Viktorin]

On 02/11/2016 11:07 AM, Nick Coghlan wrote:
> On 11 February 2016 at 19:59, Victor Stinner <victor.stinner at gmail.com> wrote:
>> 2016-02-11 9:11 GMT+01:00 Georg Brandl <g.brandl at gmx.net>:
>>> On 02/11/2016 12:04 AM, Victor Stinner wrote:
>>>> It looks like the implementation https://bugs.python.org/issue26331
>>>> only changes the Python parser.
>>>>
>>>> What about other functions converting strings to numbers at runtime
>>>> like int(str) and float(str)? Paul also asked for Decimal(str).
>>>
>>> I added these as "Open Questions" to the PEP.
>>
>> Ok nice. Now another question :-)
>>
>> Would it be useful to add an option to repr(int) and repr(float), or a
>> formatter to int.__format__() and float.__float__() to add an
>> underscore for thousands.
> 
> Given that str.format supports a thousands separator:
> 
>>>> "{:,d}".format(100000000)
> '100,000,000'
> 
> it might be reasonable to permit "_" in place of "," in the format specifier.
> 
> However, I'm not sure when you'd use it aside from code generation,
> and you can already insert the thousands separator and then replace
> "," with "_".

It would make "SI style" [0] numbers a little bit more straightforward
to generate, since the order of operations wouldn't matter.
Currently it's:

    "{:,}".format(1234.5678).replace(',', ' ').replace('.', ',')

Also it would make numbers with decimal comma and dot as separator a bit
easier to generate. Currently, that's (from PEP 378):

    format(n, "6,f").replace(",", "X").replace(".", ",").replace("X", ".")

[0] https://en.wikipedia.org/wiki/Decimal_mark#Examples_of_use