django的form验证机制

今天遇到了一个奇怪的问题，django中formview一直返回200，但是却没有执行form_valid方法，然后在其中加了一个form_invalid方法：

class StudentRegisterMixin(generic.FormView):                                    
    def get_form_class(self):                                                      
                                                                                 
    def get_success_url(self):                                                           
                                                                                 
    def form_valid(self, form):

def form_invalid(self, form):                                                
        print "form is not valid, errors are: "                                  
        print form.errors                                                        
        return http.HttpResponse(form.errors)                                    
把这个错误打印出来，然后发现总是一个地区district的域有错误，经过检查，发现是前端传过来的数据是一个字符0，这样form验证的时候不会把它当作空，

而district的域定义是这样：

district = forms.ModelChoiceField(                                           
          required=False,                                                          
          queryset=core_models.District.objects.all(),                             
      )

也就是说他有一个queryset，这个queryset会根据前端传过来的值到数据库中去找，找到了才会把其加入到cleaned_data中

而django form验证的机制是，先调用clean（）方法，得到cleaned_data,再调用clean_field方法，调用开发者对各个域附加的验证方法。。

而cleaned_data中没有的关键字是不会调用clean_field方法的。。。

这个错误找了老半天。。终于在stackoverflow和文档的帮助下解决了，解决方法是重写clean方法：

def clean(self):                                                             
          super(StudentCreationForm, self).clean()                                 
          print "cleaned_data is : ", self.cleaned_data                            
          if 'district' in self._errors:                                           
              del self._errors['district']                                         
              self.cleaned_data['district'] = None                                 
                                                                                   
          return self.cleaned_data

这样就可以忽略掉由于district而产生的错误了。