只显示了自定义模板的内容，左侧导航没有显示出来。

原因：context 漏掉了，要补上。

# 错误写法（左侧导航不显示）

    def changelist_view(self, request, extra_context=None):
        form = CsvImportForm()
        payload = {"form": form}
        return TemplateResponse(request, "admin/csv_form.html", payload)




# 正确写法：========================

    def changelist_view(self, request, extra_context=None):
        form = CsvImportForm()# 我要用的内容


        # 引入原有的内容
        context = dict(
            # Include common variables for rendering the admin template.
            self.admin_site.each_context(request),  # side nav was not loading because this was not added.
            # Anything else you want in the context...
            # key=value,

            form= form        # 加上我要用的内容

        )

        return TemplateResponse(request, "admin/csv_form.html", context)

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