def _get_single(self, section: str, option: str) -> Any:
        """Get a single-valued option.

        Performs environment substitution if the value is a string. Other types
        will be converted later as needed.
        """
        name, value = self._get(section, option)
        if isinstance(value, str):
            value = substitute_variables(value, os.environ)
        return name, value
Get a single-valued option.

        Performs environment substitution if the value is a string. Other types
        will be converted later as needed.
        
coverage/tomlconfig.py