On 9/5/2015 1:28 PM, Nathaniel Smith wrote:
> On Sep 5, 2015 9:20 AM, "Guido van Rossum" <gu...@python.org
> <mailto:gu...@python.org>> wrote:
>>
>> The processing of f-strings described by the PEP uses several phases:
>>
>> - find the end of the string (the final quote[s]) using the same
> algorithm used for all string literals
>> - expand \ escapes (e.g. \uXXXX)
>
> It might be worth calling out explicitly in the text how this works for
> what I suspect will be the most common case: embedded quoting embedded
> quotes, like
>
> f"{ table[\"foo\"] }"
>
>> - look for single {, then scan ahead to a matching } -- this skips
> matching () [] {}
>
> I assume this part of the algorithm uses the lexer and scans for
> matching tokens rather than just scanning for characters? I tried
> looking at the PEP to double check but couldn't find the relevant part.
> It's important for things like
>
> f"{ '}' }"
Actually, my current implementation doesn't use the lexer, although I
suppose it could. I'm currently manually scanning the string, keeping
track of strings and parens. To find the end of an expression, it looks
for a '!', ':', or non-doubled '}', not inside of a string or (), [], or
{}. There's a special case for '!=' so the bang isn't seen as ending the
expression.
So it does work on this f-string:
>>> f"{ '}' }"
'}'
Although by using the lexer, I'd be worried about multiple levels of
escaping. I'd have to give it some thought.
>> - look for optional !a,!r,!s and !<spec> inside each {...} pair
>> - take what's left, enclose it in (), parse as expression
>
> In fact maybe this whole list that Guido wrote should be copied into the
> PEP; it's a good summary :-).
I think the PEP does say these things, but maybe it could be tightened up.
Eric.
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