Pylint plugin that checks for non f-string literals, with format() style string interpolation

fstring_checker.py

import re
import tokenize

import astroid

from pylint.checkers import BaseChecker, BaseTokenChecker
from pylint.interfaces import IAstroidChecker, ITokenChecker


FORMAT_PATT = re.compile(r'\$\{.*?}')


class FStringChecker(BaseTokenChecker):
    """
    Check that string literals containing interpolation are an f-string.
    """

    __implements__ = ITokenChecker

    name = 'fstring-literal'
    msgs = {
        'W0001': (
            'Contains string interpolation, but is not an f-string.',
            'non-fstring-literal',
            'All interpolated string literals should be f-strings.'
        ),
    }

    def process_tokens(self, tokens):
        for tok_type, token, (start_row, _), _, _ in tokens:
            if tok_type == tokenize.STRING:
                self._process_string_token(token, start_row)

    def _process_string_token(self, token, start_row):
        if token[0] == 'f':
            return

        if not FORMAT_PATT.search(token):
            return

        self.add_message(
                'non-fstring-literal',
                line=start_row,
        )


def register(linter):
    linter.register_checker(FStringChecker(linter))

fstring_checker_demo.py

"""A demonstration file for fstring_checker.py"""

WITH_F = 'foo'

print('${without_f}')
print('${without_f_ignored}') # pylint: disable=non-fstring-literal
print(f'${WITH_F}')

Call it with

$ PYTHONPATH=. pylint --load-plugins fstring_checker fstring_checker_demo.py 
No config file found, using default configuration
************* Module fstring_checker_demo
W:  5, 0: Contains string interpolation, but is not an f-string. (non-fstring-literal)

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Your code has been rated at 7.50/10 (previous run: 5.00/10, +2.50)