Django admin decorator to create a confirmation form action, like the default delete action works

admin.py

from .models import Post, Category
from .decorators import action_form


class PostCategoryForm(forms.Form):
    title = 'Update category for the selected posts'
    myfile = forms.FileField()
    category = forms.ModelChoiceField(queryset=Category.objects.all())


@admin.register(Post)
class PostAdmin(admin.ModelAdmin):
    actions = ['change_category']
  
    @action_form(PostCategoryForm)
    def change_category(self, request, queryset, form):
        category = form.cleaned_data['category']
        return queryset.update(category=category)

decorators.py

import functools

from django.contrib.admin import helpers
from django.template.response import TemplateResponse


def action_form(form_class=None):
    def decorator(func):
        @functools.wraps(func)
        def wrapper(self, request, queryset):
            form = form_class()

            if 'confirm' in request.POST and request.POST:
                form = form_class(request.POST, request.FILES)
                if form.is_valid():
                    obj_count = func(self, request, queryset, form)
                    self.message_user(request, '%s objects updated' % obj_count)
                    return None

            context = dict(
                self.admin_site.each_context(request),
                title=form_class.title,
                action=func.__name__,
                opts=self.model._meta,
                queryset=queryset, form=form,
                action_checkbox_name=helpers.ACTION_CHECKBOX_NAME)

            return TemplateResponse(request, 'admin/form_action_confirmation.html', context)

        wrapper.short_description = form_class.title

        return wrapper

    return decorator

form_action_confirmation.html

{% extends "admin/base_site.html" %}
{% load i18n l10n admin_urls %}

{% block bodyclass %}{{ block.super }} app-{{ opts.app_label }} model-{{ opts.model_name }} delete-confirmation
  delete-selected-confirmation{% endblock %}

{% block breadcrumbs %}
  <div class="breadcrumbs">
    <a href="{% url 'admin:index' %}">{% trans 'Home' %}</a>
    &rsaquo; <a href="{% url 'admin:app_list' app_label=opts.app_label %}">{{ opts.app_config.verbose_name }}</a>
    &rsaquo; <a href="{% url opts|admin_urlname:'changelist' %}">{{ opts.verbose_name_plural|capfirst }}</a>
    &rsaquo; {{ title }}
  </div>
{% endblock %}

{% block content %}
  <ul style="padding: 0">
    {% for object in queryset.all %}
      <li style="list-style: none; float: left; margin: 5px">
        {{ object }}
      </li>
    {% endfor %}
  </ul>
  <hr>
  <br>
  <form action="" method="post" enctype="multipart/form-data">{% csrf_token %}
    <fieldset class="module aligned">
      {% for obj in queryset.all %}
        <input type="hidden" name="{{ action_checkbox_name }}" value="{{ obj.pk|unlocalize }}"/>
      {% endfor %}
      <div class="form-row">
        {{ form }}
      </div>
    </fieldset>
    <div class="submit-row">
      <input type="hidden" name="action" value="{{ action }}"/>
      <input type="submit" name="confirm" value="{% trans "Confirm" %}"/>
      <a href="#" onclick="window.history.back(); return false;"
         class="button cancel-link">{% trans "No, take me back" %}</a>
    </div>
  </form>
{% endblock %}

Thanks for the inspiration!

I'm missing something, after I hit the confirmation button, it does not return to the verification in the decorator, why?

The admin.py part is not clear; is it possible to show a simple worked example for just changing a single, unlinked model - for example, updating basic user information?

For supporting file uploads in forms, use:

{# form_action_confirmation.html #}
{# enctype required for file support, won't affect submissions except POST body format, https://stackoverflow.com/a/33891429/1960509 #}
<form action="" method="post" enctype="multipart/form-data">{% csrf_token %}

# decorators.py
# https://docs.djangoproject.com/en/4.0/topics/http/file-uploads/#basic-file-uploads
form = form_class(request.POST, request.FILES)

@twolfson thank you, I added the lines

Thank you so much!!

Unfortunately, after confirmation inside by confirmation button, I'm facing an issue, that the method (decorated one) is not being triggered so wanted actions does not run. By debugging, it can be seen, that the code is not reached after the confirmation. Do you face the same issue?

For those not getting to the confirmation: You might have dropped the {% for obj in queryset.all %} loop, or the hidden "action" field, or added a value as the form action. All should stay as-is for the admin action view to be called again after the submit button is clicked.

I do same your instructions but exist values not autofill in confirmation form, right?