[Python-Dev] Easier debugging with f-strings

Eric V. Smith eric at trueblade.com
Tue May 7 21:23:35 EDT 2019

On 5/7/19 8:39 PM, Steven D'Aprano wrote:
> Disclaimer: this topic seems to have been split over at least two issues
> on the bug tracker, a Python-Ideas thread from 2018, Discourse (I
> think...) and who knows what else. I haven't read it all, so excuse me
> if I'm raising something already discussed.

Yeah, it's all over the place. At this stage, I think discussing it here 
is best.

> On Mon, May 06, 2019 at 08:39:41PM -0400, Eric V. Smith wrote:
>> After that lightning talk, Larry and I talked about it some more, and
>> for a number of reasons decided that it would make more sense if the
>> syntax used an = sign. So we came up with f"{foo=}", which would also
>> produce "foo='Hello'".
>> The reasons for the change are:
>> - Having '=' in the expression is a better mnemonic than !d.
>> - By not using a conversion starting with !, we can compose = with the
>> existing ! conversions, !r, !s, and the rarely used !a.
>> - We can let the user have a little more control of the resulting string.
> You're going to hate me for bike-shedding, but I really don't like using
> = as a defacto unary postfix operator. To me, spam= is always going to
> look like it was meant to be spam==<something> and the second half got
> accidentally deleted.
> I don't have a better suggestion, sorry.

I think = makes a great mnemonic for what's happening. I realize it 
looks a little funny at first, but just wait until you start confusing 
it with := (hah!). Everyone else I've discussed it with likes =.

> In an earlier draft, back when this was spelled !d, you specifically
> talked about whitespace. Does this still apply?

Yes, it does.

>     spam = 42
>     f'{spam=}'  # returns 'spam=42'
>     f'{spam =}'  # returns 'spam =42'
>     f'{spam = }'  # returns 'spam = 42' I guess?

All correct.

>     f'{spam+1=}'  # returns 'spam+1=41' I guess?

I'm not proposing to redefine addition by one, so this is actually 
'spam+1=43' :)


More information about the Python-Dev mailing list