code: def _app(self):
          
          if self.app is not None:
              return self.app
  
          ctx = _request_ctx_stack.top
          if ctx is not None:
              return ctx.app
  
          try:
              from flask import _app_ctx_stack
              app_ctx = _app_ctx_stack.top
              if app_ctx is not None:
                  return app_ctx.app
          except ImportError:
              pass
  
          raise RuntimeError('assets instance not bound to an application, '+
                              'and no application in current context')
docs: The application object to work with; this is either the app
          that we have been bound to, or the current application.