Django错误:“unicode”对象不可调用

我试图从django网站上学习django教程,但我遇到了一个问题:我必须添加我的 __unicode__ 方法返回到我的模型类,但每当我尝试返回该模型的对象时,会得到以下错误:

in __unicode__
    return self.question()
TypeError: 'unicode' object is not callable

我对python很陌生,对django也很陌生,我真的看不到我在这里错过了什么,如果有人能指出的话,我会非常感激。一点点代码:

我的模型

# The code is straightforward. Each model is represented by a class that subclasses django.db.models.Model. Each model has a number of 
# class variables, each of which represents a database field in the model.

from django.db import models

    class Poll(models.Model):
        question = models.CharField(max_length=200)
        pub_date = models.DateTimeField('date published') 

        def __unicode__(self):
            return self.question


    class Choice(models.Model):
        poll = models.ForeignKey(Poll) 
        choice = models.CharField(max_length=200) 
        votes = models.IntegerField()

        def __unicode__(self):
            return self.choice()

在交互式shell中:

from pysite.polls.models import Poll, Choice
Poll.objects.all()