[issue23602] Implement __format__ for Fraction

[Tuomas Suutari]

Tuomas Suutari added the comment:

On 30 March 2015 at 13:49, Stefan Krah wrote:
> Regarding Decimal:
>
>   1) The context precision isn't used for formatting.  If you have
>      another reason for proposing the optional context argument for
>      dec_format(), please open another issue.

Yes, context precision isn't, but context rounding mode is. That's why
I wanted to use a known context rather than the current (thread-local)
context.

But yes, I also thought that maybe the Decimal.__format__ changes
should go to another issue, which my implementation would then depend
on though. It wouldn't be too bad if Py and C version of
Decimal.__format__ had same interface. What do you think?

>   2) There's another problem: The mythical DefaultContext (which
>      acts as a template for creating new contexts) affects not only
>      new thread-local contexts, but also a new Context()!
>
>      In my opinion this is something we should change: The mechanism
>      is fine for thread-local contexts, but Context() should behave
>      like a pure function.

I don't understand what do you mean with this. Is this something that
I'm doing wrong in my patch or just another (related?) issue?

>   3) The double rounding issues are more tricky than it might seem;
>      if we use Decimal for this, perhaps direct support in the module
>      would be the cleanest option.

What double rounding issues you're referring to?

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