flake8 checker for error messages showing up unrendered in the traceback

flake8_errmsg.py

#!/usr/bin/env python

"""
Flake8 checker for raw literals inside raises.

Requires Python 3.10. This can be run without Flake8 as well. Try:

git ls-files '*.py' | xargs python3 plugins/flake8_errmsg.py

Local use:
[flake8:local-plugins]
extension =
    EM1 = flake8_errmsg:ErrMsgASTPlugin
paths =
    ./plugins/
"""

from __future__ import annotations

import ast
import sys
import traceback
from pathlib import Path
from typing import Iterator, NamedTuple


class Flake8ASTErrorInfo(NamedTuple):
    line_number: int
    offset: int
    msg: str
    cls: type  # unused


class Visitor(ast.NodeVisitor):
    def __init__(self):
        self.errors: list[Flake8ASTErrorInfo] = []

    def visit_Raise(self, node):
        match node.exc:
            case ast.Call(args=[ast.Constant(value=str()), *_]):
                msg = "EM101 exception must not use a string literal, assign to variable first"
                self.errors.append(
                    Flake8ASTErrorInfo(node.lineno, node.col_offset, msg, type(self))
                )
            case ast.Call(args=[ast.JoinedStr(), *_]):
                msg = "EM102 exception must not use a f-string literal, assign to variable first"
                self.errors.append(
                    Flake8ASTErrorInfo(node.lineno, node.col_offset, msg, type(self))
                )
            case _:
                pass


class ErrMsgASTPlugin:
    name = "flake8_errmsg"
    version = "0.1.0"

    def __init__(self, tree: ast.AST) -> None:
        self._tree = tree

    def run(self) -> Iterator[Flake8ASTErrorInfo]:
        visitor = Visitor()
        visitor.visit(self._tree)
        yield from visitor.errors


def main(path: str) -> None:
    code = Path(path).read_text(encoding="utf-8")

    try:
        node = ast.parse(code)
    except SyntaxError as e:
        e.filename = path
        print("Traceback:")
        traceback.print_exception(e, limit=0)
        raise SystemExit(1) from None

    plugin = ErrMsgASTPlugin(node)
    for err in plugin.run():
        print(f"{path}:{err.line_number}:{err.offset} {err.msg}")


if __name__ == "__main__":
    for item in sys.argv[1:]:
        main(item)