On Sat, Aug 10, 2019, at 19:54, Glenn Linderman wrote:
> Because of the "invalid escape sequence" and "raw string" discussion,
> when looking at the documentation, I also noticed the following
> description for f-strings:
>
> > Escape sequences are decoded like in ordinary string literals (except when
> > a literal is also marked as a raw string). After decoding, the grammar for
> > the contents of the string is: followed by lots of stuff, followed by
> > Backslashes are not allowed in format expressions and will raise an
> error: f"newline: {ord('\n')}" # raises SyntaxError
> What I don't understand is how, if f-strings are processed AS
> DESCRIBED, how the \n is ever seen by the format expression.
>
> The description is that they are first decoded like ordinary strings,
> and then parsed for the internal grammar containing {} expressions to
> be expanded. If that were true, the \n in the above example would
> already be a newline character, and the parsing of the format
> expression would not see the backslash. And if it were true, that would
> actually be far more useful for this situation.
>
> So given that it is not true, why not? And why go to the extra work of
> prohibiting \ in the format expressions?
AIUI there were strong objections to the "AS DESCRIBED" process (which would
require almost all valid uses of backslashes inside to be doubled, and would
incidentally leave your example *still* a syntax error), and disallowing
backslashes is a way to pretend that it doesn't work that way and leave open
the possibility of changing how it works in the future without breaking
compatibility.
The only dubious benefit to the described process with backslashes allowed
would be that f-strings (or other strings, in the innermost level) could be
infinitely nested as f'{f\'{f\\\'{...}\\\'}\'}', rather than being hard-limited
to four levels as f'''{f"""{f'{"..."}'}"""}'''
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