def ensure_filename(self, option):
        """Ensure that 'option' is the name of an existing file."""
        self._ensure_tested_string(
            option, os.path.isfile, "filename", "'%s' does not exist or is not a file"
        )
Ensure that 'option' is the name of an existing file.
product/gradle-plugin/src/main/python/setuptools/_distutils/cmd.py