我的django表单没有捕获ValidationErrors

Question

所以我有一个自定义表单字段，它基本上检查我的数据库以查看给定值（凭证代码）是否有效。我有 forms.py

class VoucherField(CharField):
    def to_python(self, value):
        if not value:
            return ""
        return value

    def validate(self, value):
        super(CharField, self).validate(value)

        vouchers = Vouchers.objects[0]
        if not value in vouchers.valid:
            raise ValidationError('This voucher is invalid or has been used already.')


class PurchaseForm(Form):
    voucher = VoucherField(required=True)    
    ...

和views.py

def orderHandler(request):
    form = PurchaseForm(request.POST)
    if form.is_valid():
        # Processes
    else:
        # Return errors  

问题是form.is_valid（）行不断导致Django抛出 ValidationError 而不是捕获它并将消息保存到form.errors。我不确定为什么会发生这种情况