On Oct 4, 2018, at 6:24 PM, Tim Peters <tim.peters@gmail.com> wrote:

> Note that transforming
>     {EXPR!d:FMT}
> into
>     EXPR={repr(EXPR):FMT}
> is actually slightly more involved than transforming it into
> so I don't buy the argument that the original idea is simpler.  More
> magical and less useful, yes ;-)

[Eric V. Smith]
Actually, my proposal is to apply FMT to the entire result of 
EXPR={repr(EXPR)}, not just the repr(EXPR) part. I'm not sure either is
particularly useful.

I think I’ve come around to Tim’s way of thinking on this. If the format spec is basically useless (which I think it would be if it applied to either the whole string or to the repr of the value), then what’s the point of allowing it? I also want the 90% case to be addressed by just plain repr. 

I did consider (and implement) a version that prohibited format specs, but it seemed wrong to remove a feature that might have some clear use. 

So, my compromise is:

- f"{expr!d}" expands to f"expr={repr(expr)}", but 

- f"{expr!d:spec} expands to f"expr={format(expr, spec)}"

There’s some more discussion here:


As Paul Moore says on that issue, this is explained as: use repr, unless you specify a format spec, then use format. 

BTW, I checked, and I've never used !r, !s, or !a.  So the idea that the format could apply to a string - when EXPR itself doesn't evaluate to a string - is simply foreign to me.  I suppose it's natural to people who do use ![rsa] all the time - if such people exist ;-)

Especially when debugging and logging, I use !r all the time.