[Python-ideas] %-formatting with Decimals

[Nick Coghlan]

On 12 Mar 2014 07:51, <random832 at fastmail.us> wrote:
>
> On Tue, Mar 11, 2014, at 16:25, Ethan Furman wrote:
> > > Is % formatting not going to be updated to support new features
> >
> > No.
> >
> > > or fix bugs like this?
> >
> > It's not a bug.  Decimal is not float.
>
> The bug to which I was referring involves not calling __format__ on
> whatever object is passed in as str.format does - Decimal not being
> float is no impediment to str.format and Decimal.__format__ accepting
> the same format strings that float.__format__ does.
>
> > >  Is it going to be deprecated?
> >
> > No.  %-formatting has its place.
>
> What place is that?
>
>     There should be one-- and preferably only one --obvious way to do
>     it.
>
> We've got two syntaxes, and two functions, that do _almost_ the same
> thing, but with two completely different microlanguages. Why?

Start by reading PEP 3101, which added str.format to address the
limitations of mod-formatting.

For new users, the recommendation is unconditional: use str.format.

mod-formatting is now a power tool for optimisation, consistency with C
implementations, templating format strings without excessive escaping and
(the main reason it still exists), backwards compatibility (both for
existing code and existing programmers).

The original plan was to eventually deprecate mod-formatting entirely, but
not only did we discover that was harder than expected, we also found it
was actually incredibly useful in some cases to have a formatting system
available that relied on coercion rather than polymorphism, since it could
be faster, was more predictable and also more easily mapped to other
languages like C (especially since Python's mod-formatting is based on C's
printf syntax).

That's why the binary interpolation proposal for 3.5 now *only* proposes
restoring mod-formatting for the binary domain, leaving str.format as a
text domain only operation.

Cheers,
Nick.

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