'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):

    environ = _get_environ(request_or_environ)
    if (environ['REQUEST_METHOD'] in ('GET', 'HEAD')):
        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')))):
            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
