[Python-Dev] [Python-ideas] Rough draft: Proposed format specifier for a thousands separator (discussion moved from python-dev)

[Eric Smith]

Raymond Hettinger wrote:
> Eric Smith pointed-out that these are already
> handled by the "n" specifier in the locale module (albiet only
> for integers).

It's supported by float, but it's just not very useful. For Decimal it's
unsupported. Maybe this isn't a distinction worth pointing out.

> Proposal I (from Nick Coghlan)
> ==============================
...
> [[fill]align][sign][#][0][width][,][.precision][type]

> Proposal II (to meet Antoine Pitrou's request)
> ==============================================
...
> [[fill]align][sign][#][0][width][T[tsep]][dsep precision][type]

I was going to suggest that since the locale name for this is 
"grouping", we use "G". But since we're not doing a general-purpose 
grouping implementation, I think "T" better says "we're doing thousands, 
not general grouping. Perhaps this should go in a rationale section if 
we opt for "T". Now that I think about it, "G" is already a valid type, 
so it wouldn't work, anyway.

>  format(1234, "8T,d")    -->     '   1,234'

For proposal 2, this case is unfortunate. Because for integers, there is
no decimal allowed in the mini-language (it's currently illegal to use
"8.1d"), you'd only ever add the thousands, but you'd always need the
"T". It would be nice to come up with a specification that would degrade
for integers such that "8,d" would give '   1,234'. Proposal 1 is much
nicer in that regard, although I definitely like the fact that the
actual characters used for DOT and COMMA can be specified with proposal 2.

Maybe you'd never really use "T,", since the comma is redundant, and 
you'd say:
  format(1234, "8Td")    -->     '   1,234'
in normal use. But "d" is also the default, so it just becomes:
  format(1234, "8T")     -->     '   1,234'

I like approach 2 in general. I'll give some thought to other, similar 
schemes which would allow "8," or "8,d" to work. I think people will 
write "8," and expect "   1,234", not an error.

Eric.