[Python-ideas] f-string "debug" conversion

[Eric V. Smith]

> On Oct 4, 2018, at 6:24 PM, Tim Peters <tim.peters at gmail.com> wrote:
> 
> [Tim]
>> 
>> > Note that transforming
>> > 
>> >     {EXPR!d:FMT}
>> > 
>> > into
>> > 
>> >     EXPR={repr(EXPR):FMT}
>> > 
>> > is actually slightly more involved than transforming it into
>> > 
>> >     EXPR={EXPR:FMT}
>> > 
>> > 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:

https://bugs.python.org/issue36774#msg341306

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. 

Eric

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