Django: How to add a custom button to admin change form page that executes an admin action?

You could take a look at the change_form_template and set it to a custom template of yours and override the response_change method:

class MyModelAdmin(admin.ModelAdmin):

    # A template for a customized change view:
    change_form_template = 'path/to/your/custom_change_form.html'

    def response_change(self, request, obj):
        opts = self.model._meta
        pk_value = obj._get_pk_val()
        preserved_filters = self.get_preserved_filters(request)

        if "_customaction" in request.POST:
            # handle the action on your obj
            redirect_url = reverse('admin:%s_%s_change' %
                               (opts.app_label, opts.model_name),
                               args=(pk_value,),
                               current_app=self.admin_site.name)
             redirect_url = add_preserved_filters({'preserved_filters': preserved_filters, 'opts': opts}, redirect_url)
             return HttpResponseRedirect(redirect_url)
        else:
             return super(MyModelAdmin, self).response_change(request, obj)

Copy the change_form.html from your site-packages/django/contrib/admin/templates/change_form.html and edit the line 44

 {% if save_on_top %}{% block submit_buttons_top %}{% submit_row %}{% endblock %}{% endif %}

to

 {% if save_on_top %}{% block submit_buttons_top %}{% custom_submit_row %}{% endblock %}{% endif %}

Also check the line:

 {% block submit_buttons_bottom %}{% submit_row %}{% endblock %}

just above the javascript block.

Then you can register a new inclusion tag somewhere in your admin.py or add it to templatetags:

@register.inclusion_tag('path/to/your/custom_submit_line.html', takes_context=True)
def custom_submit_row(context):
    """
    Displays the row of buttons for delete and save.
    """
    opts = context['opts']
    change = context['change']
    is_popup = context['is_popup']
    save_as = context['save_as']
    ctx = {
        'opts': opts,
        'show_delete_link': (
            not is_popup and context['has_delete_permission'] and
            change and context.get('show_delete', True)
        ),
        'show_save_as_new': not is_popup and change and save_as,
        'show_save_and_add_another': (
            context['has_add_permission'] and not is_popup and
            (not save_as or context['add'])
        ),
        'show_save_and_continue': not is_popup and context['has_change_permission'],
        'is_popup': is_popup,
        'show_save': True,
        'preserved_filters': context.get('preserved_filters'),
    }
    if context.get('original') is not None:
        ctx['original'] = context['original']
    return ctx

The contents of your custom_submit_line.html:

{% load i18n admin_urls %}
<div class="submit-row">
{% if show_save %}<input type="submit" value="{% trans 'Save' %}" class="default" name="_save" />{% endif %}
{% if show_delete_link %}
    {% url opts|admin_urlname:'delete' original.pk|admin_urlquote as delete_url %}
    <p class="deletelink-box"><a href="{% add_preserved_filters delete_url %}" class="deletelink">{% trans "Delete" %}</a></p>
{% endif %}
{% if show_save_as_new %}<input type="submit" value="{% trans 'Save as new' %}" name="_saveasnew" />{% endif %}
{% if show_save_and_add_another %}<input type="submit" value="{% trans 'Save and add another' %}" name="_addanother" />{% endif %}
{% if show_save_and_continue %}<input type="submit" value="{% trans 'Save and continue editing' %}" name="_continue" />{% endif %}

<input type="submit" value="{% trans 'Custom Action' %}"  name="_customaction" />

</div>

It is alot of code, but mostly copy/paste. Hope that helps.

Most people probably do this without thinking, though it wasn't clear from the answer that the admin change form should be simply extended rather than overwritten entirely.

custom_change_form.html

{% extends "admin/change_form.html" %}

{% if save_on_top %}{% block submit_buttons_top %}{% custom_submit_row %}{% endblock %}{% endif %}

{% block submit_buttons_bottom %}{% custom_submit_row %}{% endblock %}