code: def make_conditional(self, request_or_environ):
          
          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()
              if 'content-length' not in self.headers:
                  length = self.calculate_content_length()
                  if length is not None:
                      self.headers['Content-Length'] = length
              if not is_resource_modified(environ, self.headers.get('etag'), None,
                                          self.headers.get('last-modified')):
                  self.status_code = 304
          return self
docs: 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.
  
          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.