Make the response conditional to the request.  This method works
        best if an etag was defined for the response already.  The `add_etag`
        method can be used to do that.  If called without etag just the date
        header is set.

        This does nothing if the request method in the request or environ is
        anything but GET or HEAD.

        For optimal performance when handling range requests, it's recommended
        that your response data object implements `seekable`, `seek` and `tell`
        methods as described by :py:class:`io.IOBase`.  Objects returned by
        :meth:`~werkzeug.wsgi.wrap_file` automatically implement those methods.

        It does not remove the body of the response because that's something
        the :meth:`__call__` function does for us automatically.

        Returns self so that you can do ``return resp.make_conditional(req)``
        but modifies the object in-place.

        :param request_or_environ: a request object or WSGI environment to be
                                   used to make the response conditional
                                   against.
        :param accept_ranges: This parameter dictates the value of
                              `Accept-Ranges` header. If ``False`` (default),
                              the header is not set. If ``True``, it will be set
                              to ``"bytes"``. If ``None``, it will be set to
                              ``"none"``. If it's a string, it will use this
                              value.
        :param complete_length: Will be used only in valid Range Requests.
                                It will set `Content-Range` complete length
                                value and compute `Content-Length` real value.
                                This parameter is mandatory for successful
                                Range Requests completion.
        :raises: :class:`~werkzeug.exceptions.RequestedRangeNotSatisfiable`
                 if `Range` header could not be parsed or satisfied.

def make_conditional(
        self, request_or_environ, accept_ranges=False, complete_length=None
    ):
        """Make the response conditional to the request.  This method works
        best if an etag was defined for the response already.  The `add_etag`
        method can be used to do that.  If called without etag just the date
        header is set.

        This does nothing if the request method in the request or environ is
        anything but GET or HEAD.

        For optimal performance when handling range requests, it's recommended
        that your response data object implements `seekable`, `seek` and `tell`
        methods as described by :py:class:`io.IOBase`.  Objects returned by
        :meth:`~werkzeug.wsgi.wrap_file` automatically implement those methods.

        It does not remove the body of the response because that's something
        the :meth:`__call__` function does for us automatically.

        Returns self so that you can do ``return resp.make_conditional(req)``
        but modifies the object in-place.

        :param request_or_environ: a request object or WSGI environment to be
                                   used to make the response conditional
                                   against.
        :param accept_ranges: This parameter dictates the value of
                              `Accept-Ranges` header. If ``False`` (default),
                              the header is not set. If ``True``, it will be set
                              to ``"bytes"``. If ``None``, it will be set to
                              ``"none"``. If it's a string, it will use this
                              value.
        :param complete_length: Will be used only in valid Range Requests.
                                It will set `Content-Range` complete length
                                value and compute `Content-Length` real value.
                                This parameter is mandatory for successful
                                Range Requests completion.
        :raises: :class:`~werkzeug.exceptions.RequestedRangeNotSatisfiable`
                 if `Range` header could not be parsed or satisfied.
        """
        environ = _get_environ(request_or_environ)
        if environ["REQUEST_METHOD"] in ("GET", "HEAD"):
            # if the date is not in the headers, add it now.  We however
            # will not override an already existing header.  Unfortunately
            # this header will be overriden by many WSGI servers including
            # wsgiref.
            if "date" not in self.headers:
                self.headers["Date"] = http_date()
            accept_ranges = _clean_accept_ranges(accept_ranges)
            is206 = self._process_range_request(environ, complete_length, accept_ranges)
            if not is206 and not is_resource_modified(
                environ,
                self.headers.get("etag"),
                None,
                self.headers.get("last-modified"),
            ):
                if parse_etags(environ.get("HTTP_IF_MATCH")):
                    self.status_code = 412
                else:
                    self.status_code = 304
            if (
                self.automatically_set_content_length
                and "content-length" not in self.headers
            ):
                length = self.calculate_content_length()
                if length is not None:
                    self.headers["Content-Length"] = length
        return self