On 8/14/2019 8:09 AM, Random832 wrote:
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'{"..."}'}"""}'''
Sure. I am just pointing out (and did so in the issue I created for documentation as well), that the documentation does not currently correctly describe the implemenation, which is misleading to the user. While I have opinions on how things could work better, my even stronger opinion is that documentation should *accurately* describe how things work, even if it how it works is more complex than it should be.