[Python-ideas] Let’s make escaping in f-literals impossible

[Franklin? Lee]

On Aug 20, 2016 1:32 PM, "Eric V. Smith" <eric at trueblade.com> wrote:
>
> On 8/19/2016 2:57 PM, Guido van Rossum wrote:
>>
>> I don't think we should take action now.
>>
>> Would it make sense, as a precaution, to declare the PEP provisional for
>> one release? Then we can develop a sense of whether the current approach
>> causes real problems.
>>
>> We could also emit some kind of warning if the expression part contains
>> an escaped quote, since that's where a potential change would cause
>> breakage. (Or we could leave that to the linters.)
>
>
> If anything, I'd make it an error to have any backslashes inside the
brackets of an f-string for 3.6. We could always remove this restriction at
a later date.
>
> I say this because as soon as f-strings make it into the wild, we're
going to have a hard time breaking code in say 3.7 by saying "well, we told
you that f-strings might change".
>
> Although frankly, other than be executive fiat (which I'm okay with), I
don't see us ever resolving the issue if f-strings are strings first, or if
the brackets put you into "non-string mode". There are good arguments on
both sides.
>
> Moving to the implementation details, I'm not sure how easy it would be
to even find backslashes, though. IIRC, backslashes are replaced early,
before the f-string parser really gets to process the string. It might
require a new implementation of the f-string parser independent of regular
strings, which I likely would not have time for before beta 1. Although
since this would be a reduction in functionality, maybe it doesn't have to
get done by then.
>
> I also haven't thought of how this would affect raw f-strings.
>
> In any event, I'll take a look at adding this restriction, just to get an
estimate of the magnitude of work involved. The easiest thing to do might
be to disallow backslashes in any part of an f-string for 3.6, although
that seems to be going too far.
>
>
> Eric.

Speaking of which, how is this parsed?
        f"{'\n'}"
If escape-handling is done first, the expression is a string literal
holding an actual newline character (normally illegal), rather than an
escape sequence which resolves to a newline character.

If that one somehow works, how about this?
        f"{r'\n'}"

I guess you'd have to write one of these:
        f"{'\\n'}"
        f"{'''\n''')"
        rf"{'\n'}"
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