[Python-3000] Format specifier proposal

[Andrew James Wade]

On Tue, 14 Aug 2007 21:12:32 -0500
Ron Adam <rrr at ronadam.com> wrote:

> 
> 
> Greg Ewing wrote:
> > Guido van Rossum wrote:
> >> On 8/13/07, Andrew James Wade <andrew.j.wade at gmail.com> wrote:
> >>
> >>> On Mon, 13 Aug 2007 20:53:26 -0700
> >>> "Guido van Rossum" <guido at python.org> wrote:
> > 
> >>>> I propose that their
> >>>> __format__ be defined like this:
> >>>>
> >>>>  def __format__(self, spec):
> >>>>      return self.strftime(spec)
> >>> You loose the ability to align the field then.
> > 
> > This might be a use case for the chaining of format specs
> > that Ron mentioned. Suppose you could do
> > 
> >    "{{1:spec1}:spec2}".format(x)
> > 
> > which would be equivalent to
> > 
> >    format(format(x, "spec1"), "spec2")

That would be a solution to my concerns, though that would have to be:

    "{ {1:spec1}:spec2}"

> 
> 
> What I was thinking of was just a simple left to right evaluation order.
> 
>      "{0:spec1, spec2, ... }".format(x)
> 
> I don't expect this will ever get very long.

The first __format__ will return a str, so chains longer than 2 don't
make a lot of sense. And the delimiter character should be allowed in
spec1; limiting the length of the chain to 2 allows that without escaping:

    "{0:spec1-with-embedded-comma,}".format(x)

My scheme did the same sort of thing with spec1 and spec2 reversed.
Your order makes more intuitive sense; I chose my order because I
wanted the syntax to be a generalization of formatting strings.

Handling the chaining within the __format__ methods should be all of
two lines of boilerplate per method.

-- Andrew