[Python-Dev] Changes to PEP 498 (f-strings)

[Eric V. Smith]

I found some time before beta 1 to modify the f-string code to implement 
the desired behavior: no backslashes inside the curly braces, but 
they're allowed in the literal string portions. I just checked it in.

I still need to update PEP 498, and the documentation needs updating. 
I'll create an issue for the docs once I've updated the PEP.

This is a fairly large change, because now I need to parse the f-strings 
in UTF-8, and do the decoding myself in pieces, instead of the previous 
behavior of decoding the string first and then parsing it.

I think I have tests for all of the backslash scenarios, but I'll watch 
the buildbots.

Eric.

On 8/30/2016 1:55 PM, Eric V. Smith wrote:
> After a long discussion on python-ideas (starting at
> https://mail.python.org/pipermail/python-ideas/2016-August/041727.html)
> I'm proposing the following change to PEP 498: backslashes inside
> brackets will be disallowed. The point of this is to disallow convoluted
> code like:
>
>>>> d = {'a': 4}
>>>> f'{d[\'a\']}'
> '4'
>
> In addition, I'll disallow escapes to be used for brackets, as in:
>
>>>> f'\x7bd["a"]}'
> '4'
>
> (where chr(0x7b) ==  "{").
>
> Because we're so close to 3.6 beta 1, my plan is to:
>
> 1. Modify the PEP to reflect these restrictions.
> 2. Modify the code to prevent _any_ backslashes inside f-strings.
>
> This is a more restrictive change than the PEP will describe, but it's
> much easier to implement. After beta 1, and hopefully before beta 2, I
> will implement the restrictions as I've outlined above (and as they will
> be documented in the PEP). The net effects are:
>
> a. Some code that works in the alphas won't work in beta 1. I'll
> document this.
> b. All code that's valid in beta 1 will work in beta 2, and some
> f-strings that are syntax errors in beta 1 will work in beta 2.
>
> I've discussed this issue with Ned and Guido, who are okay with these
> changes.
>
> The python-ideas thread I referenced above has some discussion about
> further changes to f-strings. Those proposals are outside the scope of
> 3.6, but the changes I'm putting forth here will allow for those
> additional changes, should we decide to make them. That's a discussion
> for 3.7, however.
>
> I'm sending this email out just to notify people of this upcoming
> change. I hope this won't generate much discussion. If you feel the need
> to discuss this issue further, please use the python-ideas thread (where
> some people are already ignoring it!).
>
> Eric.
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