[Python-Dev] Easier debugging with f-strings

Eric V. Smith eric at trueblade.com
Tue May 7 12:59:52 EDT 2019

Hi, Tim. 

Your name came up frequently as a target for this use case. I think we’ve come up with a pretty good solution. 

Now if I could only convince my clients to upgrade from 2.7! I’ve heard about these f-string things, and I think I would enjoy using them. 


> On May 7, 2019, at 12:50 PM, Tim Peters <tim.peters at gmail.com> wrote:
> [Larry Hastings <larry at hastings.org>]
>> Guido just stopped by--we're all at the PyCon 2019 dev sprints--and we had
>> a chat about it.  Guido likes it but wanted us to restore a little of the magical
>> behavior we had in "!d": now, = in f-strings will default to repr (!r), unless
>> you specify a format spec. If you specify a format spec it will always default
>> to format.  And naturally if you specify an explicit conversion function (!r !s !a)
>> it will use that.
>> This makes !f irrelevant, so we're removing it.
>> Here's the thinking: 99% of the time the user will just use {foo=}, and for that
>> you want repr.  After that, 0.99% of the time the user will want a format spec
>> that applies directly the value.  It's exceedingly unlikely that someone will
>> want a format spec, but want it to apply to repr(value) and not value itself.  So
>> that's possible (f'{foo=!r:20}').  But the default behavior is the most common
>> case at every step.
> +1.  Perfect!  I'm one of the 0.99% who will frequently use this to
> display floats, and really wants them to show as "0.99%" rather than
> "0.9913499340289%".
> BTW, that Guido person has made enough decent contributions by now
> that I think he should be asked whether he wants to become a core dev!
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