code: def start_response(self, status, headers, exc_info=None):
          
          # "The application may call start_response more than once,
          # if and only if the exc_info argument is provided."
          if self.started_response and not exc_info:
              raise AssertionError(
                  'WSGI start_response called a second '
                  'time with no exc_info.',
              )
          self.started_response = True
  
          # "if exc_info is provided, and the HTTP headers have already been
          # sent, start_response must raise an error, and should raise the
          # exc_info tuple."
          if self.req.sent_headers:
              try:
                  six.reraise(*exc_info)
              finally:
                  exc_info = None
  
          self.req.status = self._encode_status(status)
  
          for k, v in headers:
              if not isinstance(k, str):
                  raise TypeError(
                      'WSGI response header key %r is not of type str.' % k,
                  )
              if not isinstance(v, str):
                  raise TypeError(
                      'WSGI response header value %r is not of type str.' % v,
                  )
              if k.lower() == 'content-length':
                  self.remaining_bytes_out = int(v)
              out_header = ntob(k), ntob(v)
              self.req.outheaders.append(out_header)
  
          return self.write
docs: WSGI callable to begin the HTTP response.