[Python-Dev] Easier debugging with f-strings

Tim Peters tim.peters at gmail.com
Tue May 7 12:50:54 EDT 2019

[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

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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