[Python-Dev] Parsing f-strings from PEP 498 -- Literal String Interpolation (original) (raw)

Eric V. Smith eric at trueblade.com
Fri Nov 4 14:16:22 EDT 2016

On 11/4/2016 2:03 PM, Fabio Zadrozny wrote:

Using PyParserASTFromString is the easiest possible way to do this. Given a string, it returns an AST node. What could be simpler?

​I think that for implementation purposes, given the python infrastructure, it's fine, but for specification purposes, probably incorrect... As I don't think f-strings should accept: f"start {import sys; sys.versioninfo[0];} end" (i.e.: ​ PyParserASTFromString doesn't just return an expression, it accepts any valid Python code, even code which can't be used in an f-string).

Not so. It should only accept expressions, not statements:

f"start {import sys; sys.version_info[0];} end" File "", line 1 (import sys; sys.version_info[0];) ^ SyntaxError: invalid syntax

Also, you could still have a separate grammar saying how to look inside f-strings (this would make the lives of other implementors easier) even if it was a post-processing step as you're doing now.

Yes. I've contemplated exposing the f-string scanner. That's the part that returns expressions (as strings) and literal strings. I realize that won't help 3.6. Nice... As a note, just for the record, my own interest on f-strings is knowing how exactly they are parsed for providing a preview of PyDev with syntax highlighting and preliminary support for f-strings (which at the very minimum besides syntax highlighting for the parts of f-strings should also show syntax errors inside them).

I understand there's a need to make the specification more rigorous. Hopefully we'll get there.

Eric.

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