Vermin reports 3.6 instead of 3.8 on files with self-documenting f-strings

[lou1306]

Describe the bug
Vermin fails to detect self-documenting f-string syntax, so its result may not accurately report the minimum Python version required to run a script. Specifically, these f-strings are a Python 3.8 feature, but Vermin reports 3.6.

To Reproduce
Run Vermin on a file containing a self-documenting f-string. This can be as easy as the following script:

a = 10
print(f"{a=})"

Vermin reports Python 3.6 as the minimum required version, but running such code on Python 3.6 or 3.7 will fail with SyntaxError: invalid syntax.

Expected behavior
Vermin should report 3.8 as the minimum required version to run the code.

Environment (please complete the following information):

• Vermin version: 1.2.2

Additional context
This is likely related to issue #39. From that discussion it seems ast is not the best way to detect such idioms, because it is essentially syntactic sugar for f"a={a}".

[netromdk]

Thanks for reaching out. If you read the caveats section of the readme, it says:

Self-documenting fstrings detection has been disabled by default because the built-in AST cannot
distinguish f'{a=}' from f'a={a}', for instance, since it optimizes some information away
(#39). And this incorrectly marks some source code as using fstring self-doc when only using general fstring. To enable (unstable) fstring self-doc detection, use --feature fstring-self-doc.

The project uses the built-in lexer and parser that Python itself uses such that it is always in line with the features of the language - and how to parse them. Unfortunately, it optimizes away important information in the case of self-documenting fstrings, which is why the feature is marked as unstable because it cannot be precise enough without the information.

It's unfortunate but it does not make sense, for the project, to change lexer and parser. That would also incur additional dependencies to the project that changes the minimum version of Python needed to run it.

If you have the following code:

a = 10
print(f"{a=}")

And run Vermin with the feature enabled, you'll get:

% ./vermin.py -vv --feature fstring-self-doc test.py
Detecting python files..
Analyzing using 8 processes..
!2, 3.8      test.py
  f-strings requires !2, 3.6
  print(expr) requires 2.0, 3.0
  self-documenting fstrings requires !2, 3.8

Minimum required versions: 3.8
Incompatible versions:     2

I hope that is sufficient for your cases.

[lou1306]

Sorry, I totally missed the caveats. I will use --feature fstring-self-doc in the future. Thanks!

[netromdk]

Great. No worries!