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

Nick Coghlan ncoghlan at gmail.com
Fri Mar 13 12:57:06 CET 2009

Eric Smith wrote:
> 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.

Given that PEP 3101 deliberately exposed the underlying Formatter class
to support more flexible expansions to formatting, I think trying to
build that level of flexibility into the formatting mini-language itself
would be a mistake.

So step 1 would be to add the "," or "'" to the format string to
indicate the use of separators. While both violate the rule of syntax
not looking like grit on Tim's monitor, the latter has the virtues of
not only being harder to confuse with a period, but also being
consistent with the printf() grouping flag as defined in the Single Unix
Specification v2 (as noted by JYK earlier in the thread and described in
more detail at [1]).

Then step 2 (if deemed appropriate) would be to expose the mini-language
parser in a way that makes it easier to write a custom format_field()
method in a Formatter subclass without having to completely duplicate
the existing parser's functionality.

Step 1 makes sense even if step 2 is never taken. Step 2 also makes
sense even if step 1 is never taken. So I don't think the fact that step
2 is probably a good idea should be used to hold up PEP 378, nor do I
think PEP 378 should be complicated by locale related issues that are
better handled by making it easier to write a custom format_field()
method in a Formatter subclass.


[1] http://linux.die.net/man/3/printf

P.S. This post is really just an elaboration of this comment from JYK:

"""I'm not against Raymond's proposal, just against doing a *bad* job of
making it work in multiple locales. Locale conventions can be complex,
and are going to be best represented outside the format string."""

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

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