code: def request_headers(self, request):
          
          headers = {}
  
          allow_origin = self.allow_origin(request)
          if allow_origin:
              headers = dict_filter({
                  'Access-Control-Allow-Origin': allow_origin,
                  'Access-Control-Allow-Credentials': {True: 'true', False: 'false'}.get(self.allow_credentials),
                  'Access-Control-Expose-Headers': ', '.join(self.expose_headers) if self.expose_headers else None,
              })
  
          return headers
docs: Generate standard request headers