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'{"..."}'}"""}'''