django form field which does have a value throw can not be null error

Question

detail code:

form:

class TrainingExpForm(BaseModelForm):
    company_type = forms.ChoiceField(choices=COMPANY_TYPE_CHOICE)
    company = forms.CharField(label=u'your company', required=True)

    def __init__(self, *args, **kwargs):
        super(TrainingExpForm, self).__init__(*args, **kwargs)
        self.fields['exp_type'].initial = 'T'
        # redefine order, because extra field 'company_type' can not write in Meta.fields
        self.fields.keyOrder = ('exp_type', 'company_type', 'company', 'name', 'start_date', 'end_date',  'description')

    def clean_exp_type(self):
        exp_type = self.cleaned_data.get('exp_type')
        if not exp_type:
            self.cleaned_data['exp_type'] = self.initial['exp_type']

    class Meta:
        model = Experience
        fields = ('exp_type', 'company', 'name', 'start_date', 'end_date',  'description')
        widgets = {
            'exp_type': HiddenInput(),
        }

def get_exp_form(exp_type):
    return WorkExpForm if exp_type =='W' else TrainingExpForm

views:

def exp_manage(request, exp_type, id=None):
    if id:
        exp = Experience.objects.get(id=id)
    else:
        exp = None

    ExpForm = get_exp_form(exp_type)

    if request.method == 'POST':
        form = ExpForm(request.POST, instance=exp)
        if form.is_valid():
            return http.HttpResponseRedirect(reverse('zhemei_resume_manage'))
    else:
        form = ExpForm(instance=exp)

I have set a breakpoint at line:if form.is_valid():.

>>> request.POST.get('exp_type')
u'T'
>>> form.data.get('exp_type')
u'T'
>>> form.errors
{'exp_type': [u'can not be null']}

I can not understand what result to above .I have also tried to figure out how django form generate form.errors. But I just can't find the relevent code in django source.
Anyone can help?

Answer 1

clean_xxx method must return value of the field:

def clean_exp_type(self):
    exp_type = self.cleaned_data.get('exp_type')
    if not exp_type:
        exp_type = self.initial['exp_type']
    return exp_type