UnboundLocalError：局部变量…在分配前被引用

只需切换：

class Test(webapp.RequestHandler):    def err_user_not_found(self):        self.redirect(users.create_login_url(self.request.uri))    def get(self):   user = users.get_current_user()        # error path        if not user: self.err_user_not_found() return        # happy path        greeting = ('Hello, ' + user.nickname())        ...        template_values = {"greeting": greeting,}