python - coercing to Unicode: need string or buffer, NoneType found when rendering in django admin

Question

I have this error since a long time but can't figure it out :

Caught TypeError while rendering: coercing to Unicode: need string or buffer, NoneType found

It happens in admin when I try to add or modify on one of my models (display works fine)

This is the model:

class PS(models.Model):
    id_ps = models.IntegerField(null=True)
    client = models.ForeignKey(Client, null=True, blank=True)
    nom_du_site = models.CharField(max_length=250)
    rue_livraison = models.TextField(null=True)
    complement_adresse = models.TextField(null=True)
    code_postal_livraison = models.CharField(max_length=50, null=True)
    ville_livraison = models.CharField(max_length=200, null=True)
    pays_livraison = models.CharField(max_length=200, null=True)
    distributeur = models.CharField(max_length=50, null=True)
    tarif = models.CharField(max_length=250, null=True)
    type_tarif = models.CharField(max_length=50, null=True)
    type_installation = models.CharField(max_length=50, null=True)

    def __unicode__(self):
        return self.nom_du_site

    @property
    def pce(self):        
        try:
            return PCE.objects.filter(ps=self)[0]
        except IndexError:
            return
    #    
    def get_pce(self):
        from pce.models import PCE
        return PCE.objects.filter(ps=self).exclude(frequence='6M')
    #
    def get_pce_6m(self):
        from pce.models import PCE
        return PCE.objects.filter(ps=self,frequence='6M') 

Any idea what am i doing wrong here?

Traceback:

    Environment:

    Request Method: GET
    Request URL: http://127.0.0.1:8026/admin/ps/ps/add/
    Django Version: 1.2.5
    Python Version: 2.7.2
    Installed Applications:
    ['django.contrib.auth',
     'django.contrib.contenttypes',
     'django.contrib.sessions',
     'django.contrib.sites',
     'django.contrib.messages',
     'django.contrib.admin',
     'easy_thumbnails',
     'django_extensions',
     'contentadmin',
     'south',
     'sentry',
     'sentry.client',
     'indexer',
     'file_uploader',
     'paging',
     'pagination',
     'consommation',
     'ps',
     'pce',
     'profil',
     'analytics',
     'document']
    Installed Middleware:
    ('annoying.middlewares.StaticServe',
     'django.middleware.common.CommonMiddleware',
     'django.contrib.sessions.middleware.SessionMiddleware',
     'django.middleware.csrf.CsrfViewMiddleware',
     'django.contrib.auth.middleware.AuthenticationMiddleware',
     'django.contrib.messages.middleware.MessageMiddleware',
     'pagination.middleware.PaginationMiddleware')


    Template error:
    In template /usr/local/lib/python2.7/dist-packages/django/contrib/admin/templates/admin/includes/fieldset.html, error at line 19
       Caught TypeError while rendering: coercing to Unicode: need string or buffer, NoneType found
       9 :             {% for field in line %}


       10 :                 <div{% if not line.fields|length_is:'1' %} class="field-box{% if not field.is_readonly and field.errors %} errors{% endif %}"{% endif %}>


       11 :                     {% if not line.fields|length_is:'1' and not field.is_readonly %}{{ field.errors }}{% endif %}


       12 :                     {% if field.is_checkbox %}


       13 :                         {{ field.field }}{{ field.label_tag }}


       14 :                     {% else %}


       15 :                         {{ field.label_tag }}


       16 :                         {% if field.is_readonly %}


       17 :                             <p>{{ field.contents }}</p>


       18 :                         {% else %}


       19 :                              {{ field.field }} 


       20 :                         {% endif %}


       21 :                     {% endif %}


       22 :                     {% if field.field.field.help_text %}


       23 :                         <p class="help">{{ field.field.field.help_text|safe }}</p>


       24 :                     {% endif %}


       25 :                 </div>


       26 :             {% endfor %}


       27 :         </div>


       28 :     {% endfor %}


       29 : </fieldset>


    Traceback:
    File "/usr/local/lib/python2.7/dist-packages/django/core/handlers/base.py" in get_response
      100.                     response = callback(request, *callback_args, **callback_kwargs)
    File "/usr/local/lib/python2.7/dist-packages/django/contrib/admin/options.py" in wrapper
      288.                 return self.admin_site.admin_view(view)(*args, **kwargs)
    File "/usr/local/lib/python2.7/dist-packages/django/utils/decorators.py" in _wrapped_view
      76.                     response = view_func(request, *args, **kwargs)
    File "/usr/local/lib/python2.7/dist-packages/django/views/decorators/cache.py" in _wrapped_view_func
      78.         response = view_func(request, *args, **kwargs)
    File "/usr/local/lib/python2.7/dist-packages/django/contrib/admin/sites.py" in inner
      190.             return view(request, *args, **kwargs)
    File "/usr/local/lib/python2.7/dist-packages/django/utils/decorators.py" in _wrapper
      21.             return decorator(bound_func)(*args, **kwargs)
    File "/usr/local/lib/python2.7/dist-packages/django/utils/decorators.py" in _wrapped_view
      76.                     response = view_func(request, *args, **kwargs)
    File "/usr/local/lib/python2.7/dist-packages/django/utils/decorators.py" in bound_func
      17.                 return func(self, *args2, **kwargs2)
    File "/usr/local/lib/python2.7/dist-packages/django/db/transaction.py" in _commit_on_success
      299.                     res = func(*args, **kw)
    File "/usr/local/lib/python2.7/dist-packages/django/contrib/admin/options.py" in add_view
      909.         return self.render_change_form(request, context, form_url=form_url, add=True)
    File "/usr/local/lib/python2.7/dist-packages/django/contrib/admin/options.py" in render_change_form
      692.         ], context, context_instance=context_instance)
    File "/usr/local/lib/python2.7/dist-packages/django/shortcuts/__init__.py" in render_to_response
      20.     return HttpResponse(loader.render_to_string(*args, **kwargs), **httpresponse_kwargs)
    File "/usr/local/lib/python2.7/dist-packages/django/template/loader.py" in render_to_string
      186.     return t.render(context_instance)
    File "/usr/local/lib/python2.7/dist-packages/django/template/__init__.py" in render
      173.             return self._render(context)
    File "/usr/local/lib/python2.7/dist-packages/django/template/__init__.py" in _render
      167.         return self.nodelist.render(context)
    File "/usr/local/lib/python2.7/dist-packages/django/template/__init__.py" in render
      796.                 bits.append(self.render_node(node, context))
    File "/usr/local/lib/python2.7/dist-packages/django/template/debug.py" in render_node
      72.             result = node.render(context)
    File "/usr/local/lib/python2.7/dist-packages/django/template/loader_tags.py" in render
      125.         return compiled_parent._render(context)
    File "/usr/local/lib/python2.7/dist-packages/django/template/__init__.py" in _render
      167.         return self.nodelist.render(context)
    File "/usr/local/lib/python2.7/dist-packages/django/template/__init__.py" in render
      796.                 bits.append(self.render_node(node, context))
    File "/usr/local/lib/python2.7/dist-packages/django/template/debug.py" in render_node
      72.             result = node.render(context)
    File "/usr/local/lib/python2.7/dist-packages/django/template/loader_tags.py" in render
      125.         return compiled_parent._render(context)
    File "/usr/local/lib/python2.7/dist-packages/django/template/__init__.py" in _render
      167.         return self.nodelist.render(context)
    File "/usr/local/lib/python2.7/dist-packages/django/template/__init__.py" in render
      796.                 bits.append(self.render_node(node, context))
    File "/usr/local/lib/python2.7/dist-packages/django/template/debug.py" in render_node
      72.             result = node.render(context)
    File "/usr/local/lib/python2.7/dist-packages/django/template/loader_tags.py" in render
      62.             result = block.nodelist.render(context)
    File "/usr/local/lib/python2.7/dist-packages/django/template/__init__.py" in render
      796.                 bits.append(self.render_node(node, context))
    File "/usr/local/lib/python2.7/dist-packages/django/template/debug.py" in render_node
      72.             result = node.render(context)
    File "/usr/local/lib/python2.7/dist-packages/django/template/defaulttags.py" in render
      174.                 nodelist.append(node.render(context))
    File "/usr/local/lib/python2.7/dist-packages/django/template/loader_tags.py" in render
      139.             return self.template.render(context)
    File "/usr/local/lib/python2.7/dist-packages/django/template/__init__.py" in render
      173.             return self._render(context)
    File "/usr/local/lib/python2.7/dist-packages/django/template/__init__.py" in _render
      167.         return self.nodelist.render(context)
    File "/usr/local/lib/python2.7/dist-packages/django/template/__init__.py" in render
      796.                 bits.append(self.render_node(node, context))
    File "/usr/local/lib/python2.7/dist-packages/django/template/debug.py" in render_node
      72.             result = node.render(context)
    File "/usr/local/lib/python2.7/dist-packages/django/template/defaulttags.py" in render
      174.                 nodelist.append(node.render(context))
    File "/usr/local/lib/python2.7/dist-packages/django/template/defaulttags.py" in render
      174.                 nodelist.append(node.render(context))
    File "/usr/local/lib/python2.7/dist-packages/django/template/defaulttags.py" in render
      260.             return self.nodelist_false.render(context)
    File "/usr/local/lib/python2.7/dist-packages/django/template/__init__.py" in render
      796.                 bits.append(self.render_node(node, context))
    File "/usr/local/lib/python2.7/dist-packages/django/template/debug.py" in render_node
      72.             result = node.render(context)
    File "/usr/local/lib/python2.7/dist-packages/django/template/defaulttags.py" in render
      260.             return self.nodelist_false.render(context)
    File "/usr/local/lib/python2.7/dist-packages/django/template/__init__.py" in render
      796.                 bits.append(self.render_node(node, context))
    File "/usr/local/lib/python2.7/dist-packages/django/template/debug.py" in render_node
      72.             result = node.render(context)
    File "/usr/local/lib/python2.7/dist-packages/django/template/debug.py" in render
      91.             output = force_unicode(output)
    File "/usr/local/lib/python2.7/dist-packages/django/utils/encoding.py" in force_unicode
      66.                 s = unicode(s)
    File "/usr/local/lib/python2.7/dist-packages/django/forms/forms.py" in __unicode__
      408.         return self.as_widget()
    File "/usr/local/lib/python2.7/dist-packages/django/forms/forms.py" in as_widget
      450.         return widget.render(name, data, attrs=attrs)
    File "/usr/local/lib/python2.7/dist-packages/django/contrib/admin/widgets.py" in render
      247.         output = [self.widget.render(name, value, *args, **kwargs)]
    File "/usr/local/lib/python2.7/dist-packages/django/forms/widgets.py" in render
      447.         options = self.render_options(choices, [v

Answer 1

This error happens when you have a __unicode__ method that is a returning a field that is not entered. Any blank field is None and Python cannot convert None, so you get the error.

In your case, the problem most likely is with the PCE model's __unicode__ method, specifically the field its returning.

You can prevent this by returning a default value:

def __unicode__(self):
   return self.some_field or u'None'