def _ensure_sequence(self, mutable: bool = False) -> None:
        """This method can be called by methods that need a sequence.  If
        `mutable` is true, it will also ensure that the response sequence
        is a standard Python list.

        .. versionadded:: 0.6
        """
        if self.is_sequence:
            # if we need a mutable object, we ensure it's a list.
            if mutable and not isinstance(self.response, list):
                self.response = list(self.response)  # type: ignore
            return
        if self.direct_passthrough:
            raise RuntimeError(
                "Attempted implicit sequence conversion but the"
                " response object is in direct passthrough mode."
            )
        if not self.implicit_sequence_conversion:
            raise RuntimeError(
                "The response object required the iterable to be a"
                " sequence, but the implicit conversion was disabled."
                " Call make_sequence() yourself."
            )
        self.make_sequence()
This method can be called by methods that need a sequence.  If
        `mutable` is true, it will also ensure that the response sequence
        is a standard Python list.

        .. versionadded:: 0.6
        
src/werkzeug/wrappers/response.py