Hi, I originially posted this via google groups, which didn’t make it through to the list proper, sorry! Read it here please: https://groups.google.com/forum/#!topic/python-ideas/V1U6DGL5J1s

My arguments are basically:
  1. f-literals are semantically not strings, but expressions.
  2. Their escape sequences in the code parts are fundamentally both detrimental and superfluous (they’re only in for convenience, as confirmed by Guido in the quote below)
  3. They’re detrimental because Syntax highlighters are (by design) unable to handle this part of Python 3.6a4’s grammar. This will cause code to be highlighted as parts of a string and therefore overlooked. i’m very sure this will cause bugs.
  4. The fact that people see the embedded expressions as somehow “part of the string” is confusing.
My poposal is to redo their grammar:
They shouldn’t be parsed as strings and post-processed, but be their own thing. This also opens the door to potentially extend to with something like JavaScript’s tagged templates)

Without the limitations of the string tokenization code/rules, only the string parts would have escape sequences, and the expression parts would be regular python code (“holes” in the literal).

Below the mentioned quote and some replies to the original thread:

Guido van Rossum <guido@python.org> schrieb am Mi., 17. Aug. 2016 um 20:11 Uhr:
The explanation is honestly that the current approach is the most straightforward for the implementation (it's pretty hard to intercept the string literal before escapes have been processed) and nobody cares enough about the edge cases to force the implementation to jump through more hoops.

I really don't think this discussion should be reopened. If you disagree, please start a new thread on python-ideas.

I really think it should. Please look at python code with f-literals. if they’re highlighted as strings throughout, you won’t be able to spot which parts are code. if they’re highlighted as code, the escaping rules guarantee that most highlighters can’t correctly highlight python anymore. i think that’s a big issue for readability.

Brett Cannon <brett@python.org> schrieb am Mi., 17. Aug. 2016 um 20:28 Uhr:
They are still strings, there is just post-processing on the string itself to do the interpolation.

Sounds hacky to me. I’d rather see a proper parser for them, which of course would make my vision easy.
 
By doing it this way the implementation can use Python itself to do the tokenizing of the string, while if you do the string interpolation beforehand you would then need to do it entirely at the C level which is very messy and painful since you're explicitly avoiding Python's automatic handling of Unicode, etc.

of course we reuse the tokenization for the string parts. as said, you can view an f-literal as interleaved sequence of strings and expressions with an attached format specification.

<f'> starts the f-literal, string contents follow. the only difference to other strings is
<{> which starts expression tokenization. once the expression ends, an optional
<formatspec> follows, then a
<}> to switch back to string tokenization
this repeats until (in string parsing mode) a
<'> is encountered which ends the f-literal.

You also make it harder to work with Unicode-based variable names (or at least explain it). If you have Unicode in a variable name but you can't use \N{} in the string to help express it you then have to say "normal Unicode support in the string applies everywhere *but* in the string interpolation part".

i think you’re just proving my point that the way f-literals work now is confusing.

the embedded expressions are just normal python. the embedded strings just normal strings. you can simply switch between both using <{> and <[format]}>.

unicode in variable names works exactly the same as in all other python code because it is regular python code.

Or another reason is you can explain f-strings as "basically str.format_map(**locals(), **globals()), but without having to make the actual method call" (and worrying about clashing keys but I couldn't think of a way of using dict.update() in a single line). But with your desired change it kills this explanation by saying f-strings aren't like this but some magical string that does all of this stuff before normal string normalization occurs.

no, it’s simply the expression parts (that for normal formatting are inside of the braces of  .format(...)) are *interleaved* in between string parts. they’re not part of the string. just regular plain python code.

Cheers, and i really hope i’ve made a strong case,
philipp