[Python-Dev] PEP-498: Literal String Formatting

Paul Moore p.f.moore at gmail.com
Sun Aug 16 16:21:00 CEST 2015

On 8 August 2015 at 02:39, Eric V. Smith <eric at trueblade.com> wrote:
> Following a long discussion on python-ideas, I've posted my draft of
> PEP-498. It describes the "f-string" approach that was the subject of
> the "Briefer string format" thread. I'm open to a better title than
> "Literal String Formatting".
> I need to add some text to the discussion section, but I think it's in
> reasonable shape. I have a fully working implementation that I'll get
> around to posting somewhere this weekend.
>>>> def how_awesome(): return 'very'
> ...
>>>> f'f-strings are {how_awesome()} awesome!'
> 'f-strings are very awesome!'
> I'm open to any suggestions to improve the PEP. Thanks for your feedback.

In my view:

1. Calling them "format strings" rather than "f-strings" is sensible
(by analogy with "raw string" etc). Colloquially we can use f-string
if we want, but let's have the formal name be fully spelled out. In
particular, the PEP should use "format string".

2. By far and away the most common use for me would be things like
print(f"Iteration {n}: Took {end-start) seconds"). At the moment I use
str,format() for this, and it's annoyingly verbose. This would be a
big win, and I'm +1 on the PEP for this specific reason.

3. All of the complex examples look scary, but in practice I wouldn't
write stuff like that - why would anyone do so unless they were being
deliberately obscure? On the other hand, as I gained experience with
the construct, being *able* to use more complex expressions without
having to stop and remember special cases would be great.

4. It's easy to write print("My age is {age}") and forget the "f"
prefix. While it'll bug me at first that I have to go back and fix
stuff to add the "f" after my code gives the wrong output, I *don't*
want to see this ability added to unprefixed strings. IMO that's going
a step too far (explicit is better than implicit and all that).

5. The PEP is silent (as far as I can see) on things like whether
triple quoting (f"""...""") is allowed (I assume it is), and whether
prefixes can be combined (for example, rf'{drive}:\{path}\{filename}')
(I'd like them to be, but can live without it).

6. The justification for ignoring whitespace is weak (the motivating
case is obscure, and there are many viable workarounds). I don't think
it's worth ignoring whitespace - but I also don't think it's worth a
long discussion. Just pick an option (as you did) and go with it. So I
see no need for change here,

Apologies for the above being terse - I'm clearing a big backlog of
emails. Ask for clarification if you need it!


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