赋值错误前引用的局部变量“instance”

现在我已经阅读了所有关于这个错误的文章,我可以得出结论,我的错误是完全不同的情况。

错误: Local variable 'instance' referenced before assignment [21/Jun/2018 09:05:58] "POST /details/create/ HTTP/1.1" 400 54

下面是我初始化的代码 instance :

def create(request):
if request.method == "POST":
    try:
        params = post_data(request)

        try:
            instance = Sales_detail.objects.get(id = params.get("id",None))
            params["item"] = instance.item.pk
            params["price"] = instance.price.pk
            params["sales"] = instance.sales.pk
            detail_form = Detail_form(params, instance = instance)
        except Sales_detail.DoesNotExist:
            params["item"] = instance.item.pk
            params["price"] = instance.price.pk
            params["sales"] = instance.sales.pk
            detail_form = Detail_form(params)

        if detail_form.is_valid():
            detail_form.save()
        else:
            raise_error(detail_form.errors,True)

        return success("Details successfully saved.")
    except Exception as e:
        return error(e)
else:
    return redirect("dashboard")

我不知道我错过了什么 实例 在用它做任何事情之前初始化。