def the_callable(obj):
    return '<a href="#">link from the callable for {0}</a>'.format(obj)
the_callable.allow_tags = True


class SomeAdmin(admin.ModelAdmin):

    def the_method_in_modeladmin(self, obj):
        return ('<a href="#">link from the method of modeladmin for {0}</a>'
            .format(obj))
the_method_in_modeladmin.allow_tags = True
readonly_fields = (the_callable, 'the_method_in_modeladmin',
    'the_callable_on_object')
django admin: how to make a readonly url field clickable in change_form.html?