'str' object is not callable - SuccessMessageMixin - django

Question

I get this error in a project whith SuccessMessageMixin and and not know why. This is my code in view.py.
from django.contrib.messages.views import SuccessMessageMixin
from django.views.generic import CreateView
class CampanaNueva(SuccessMessageMixin, CreateView):
model = Campana
template_name = "licencias_campana_nueva.html"
fields = ['temporada', 'descripcion']
success_message = "a"
And Raise this error on save:
Environment:
Request Method: POST
Request URL: http://127.0.0.1:8000/licencias/editar/1
Django Version: 1.9.4
Python Version: 3.4.4
Installed Applications:
['django.contrib.admin',
'django.contrib.auth',
'django.contrib.contenttypes',
'django.contrib.sessions',
'django.contrib.messages',
'django.contrib.staticfiles',
'bootstrap3',
'home',
'widget_tweaks',
'socios',
'equipaciones',
'licencias']
Installed Middleware:
['django.middleware.security.SecurityMiddleware',
'django.contrib.sessions.middleware.SessionMiddleware',
'django.middleware.common.CommonMiddleware',
'django.middleware.csrf.CsrfViewMiddleware',
'django.contrib.auth.middleware.AuthenticationMiddleware',
'django.contrib.auth.middleware.SessionAuthenticationMiddleware',
'django.contrib.messages.middleware.MessageMiddleware',
'django.middleware.clickjacking.XFrameOptionsMiddleware',
'cc_corbelo.middleware.LoginRequiredMiddleware']
Traceback:
File "C:\Python34\lib\site-packages\django\core\handlers\base.py" in get_response
149. response = self.process_exception_by_middleware(e, request)
File "C:\Python34\lib\site-packages\django\core\handlers\base.py" in get_response
147. response = wrapped_callback(request, *callback_args, **callback_kwargs)
File "C:\Python34\lib\site-packages\django\views\generic\base.py" in view
68. return self.dispatch(request, *args, **kwargs)
File "C:\Python34\lib\site-packages\django\views\generic\base.py" in dispatch
88. return handler(request, *args, **kwargs)
File "C:\Python34\lib\site-packages\django\views\generic\edit.py" in post
279. return super(BaseUpdateView, self).post(request, *args, **kwargs)
File "C:\Python34\lib\site-packages\django\views\generic\edit.py" in post
222. return self.form_valid(form)
File "C:\Python34\lib\site-packages\django\contrib\messages\views.py" in form_valid
14. messages.success(self.request, success_message)
Exception Type: TypeError at /licencias/editar/1
Exception Value: 'str' object is not callable
I have this function working in another project without problems...

Answer 1

I got exactly this same message when I accidentally used the messages framework incorrectly in the form_valid method of a completely different view in my project.
The correct code should have been:
messages.success(request, "Deactivated product")
But what I had written instead was
messages.success = "Deactivated product"
This code doesn't work, but it doesn't cause an error on the page, either! However, as soon as I submitted a different form that used the SuccessMessageMixin, I would see the 'str' object is not callable error.
(Having a problem on one page cause an error on a completely different page was fun to debug.)