code: def _get_application(self, subdomain):
          
          with self.lock:
              app = self.instances.get(subdomain)
              if app is None:
                  app = self.create_application(subdomain=subdomain)
                  self.instances[subdomain] = app
              return app
docs: Return a WSGI application for subdomain.  The subdomain is
          passed to the create_application constructor as a keyword argument.
  
          :param subdomain: Subdomain to get or create an application with