form组件 初始化

 class MyForm(Form):

     user = fields.ChoiceField(
         # choices=((1, '上海'), (2, '北京'),),
         initial=2,
         widget=widgets.Select
     )

     def __init__(self, *args, **kwargs):
         super(MyForm,self).__init__(*args, **kwargs)  #每次生成初始化self.filed，区数据库查询
         # self.fields['user'].widget.choices = ((1, '上海'), (2, '北京'),)
         # 或
         self.fields['user'].widget.choices = models.Classes.objects.all().value_list('id','caption')