[Python-ideas] Format mini-language for lakh and crore

[Nick Coghlan]

On 29 January 2018 at 11:48, Nathaniel Smith <njs at pobox.com> wrote:
> On Sun, Jan 28, 2018 at 5:31 PM, David Mertz <mertz at gnosis.cx> wrote:
>> I actually didn't know about `locale.format("%d", 10e9, grouping=True)`.
>> But it's still much less general than having the option in the
>> f-string/.format() mini-language.  This is really about the formatted
>> string, not necessarily about the locale.  So, e.g. I'd like to be able to
>> write:
>>
>>>>> print(f"In European format x is {x:,.2f}, in Indian format it is
>>>>> {x:`.2f}")
>>
>> I don't want the format necessarily to be some pseudo-global setting, even
>> if it can get stored in thread-locals.  That said, having a locale-aware
>> symbol for delimiting numbers in the format mini-language would also not be
>> a bad thing.
>
> I don't understand the format mini-language well enough to know what
> would fit in, but maybe some way to (a) request localified formatting,

Given the example, I think a more useful approach would be to allow an
optional digit grouping specifier after the comma separator, and allow
the separator to be repeated to indicate non-uniform groupings in the
lower order digits.

If we did that, then David's example could become:

    >>> print(f"In European format x is {x:,.2f}, in Indian format it
is {x:,2,3.2f}")

The core elements of interpreting that would then be:

- digit group size specifiers are permited for both "," (decimal
display only) and "_" (all display bases)
- if no digit group size specifier is given, it defaults to 3 for
decimal and 4 for binary, octal, and hexadecimal
- if multiple digit group specifiers are given, then the last one
given is applied starting from the least significant integer digit

so "{x:,2,3.2f}" means:

- an arbitrary number of leading 2-digit groups
- 1 group of 3 digits
- 2 decimal places

It would then be reasonably straightforward to use this as a lower
level primitive to implement locale dependent formatting, as follows:

    - format in English using the locale's grouping rules [1] (either
LC_NUMERIC.grouping or LC_MONETARY.mon_grouping, as appropriate)
    - use str.translate() [2] to replace "," and "." with the locale's
thousands_sep & decimal_point or mon_thousands_sep & mon_decimal_point

[1] https://docs.python.org/3/library/locale.html#locale.localeconv
[2] https://docs.python.org/3/library/stdtypes.html#str.translate

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia