super(type, obj): obj must be an instance or subtype of type

Question

I work on a small Django app and get an error tells me, super(type, obj): obj must be an instance or subtype of type. I get it from the views.py file after introducing the function get_object_or_404. The views.py file provided below,

from django.shortcuts import render, get_object_or_404     from django.http import HttpResponse, HttpResponseRedirect      from django.views import View      from .models import URL   # function based view  def redirect_view(request, shortcode=None, *args, **kwargs):     obj = get_object_or_404(URL, shortcode=shortcode)     return HttpResponse("Hello World, the shortcode is {shortcode}".format(shortcode = obj.url))   # class based view  class ShortenerView(View):      def get(self, request, shortcode=None,  *args, **kwargs):         obj = get_object_or_404(URL, shortcode=shortcode)         return HttpResponse("Hello World 1, the shortcode is {shortcode}".format(shortcode = obj.url))      def post(self, request, *args, **kwargs):         return HttpResponse() 

the full error message is here,

TypeError at /b/p6jzbp/ super(type, obj): obj must be an instance or subtype of type Request Method: GET Request URL:    http://127.0.0.1:8000/b/p6jzbp/ Django Version: 1.11 Exception Type: TypeError Exception Value:     super(type, obj): obj must be an instance or subtype of type Exception Location: /Users/Chaklader/Documents/Projects/UrlShortener/src/shortener/models.py in all, line 18 

The line 18 in the models.py is qs_main = super(URL, self).all(*args, **kwargs) and the models.py file is here,

#  will look for the "SHORTCODE_MAX" in the settings and  #  if not found, will put the value of 15 there  SHORTCODE_MAX = getattr(settings, "SHORTCODE_MAX", 15)    class UrlManager(models.Manager):      def all(self, *args, **kwargs):         qs_main = super(URL, self).all(*args, **kwargs)         qs      = qs_main.filter(active = True)         return qs      def refresh_shortcodes(self, items = None):          qs = URL.objects.filter(id__gte=1)         new_codes = 0          if items is not None and isinstance(items, int):             qs = qs.order_by('-id')[:items]          for q in qs:             q.shortcode = create_shortcode(q)             print (q.id, " ", q.shortcode)             q.save()             new_codes += 1          return "# new codes created {id}".format(id = new_codes)   class URL(models.Model):      url         =  models.CharField(max_length = 220, )     shortcode   =  models.CharField(max_length = SHORTCODE_MAX, blank = True, unique = True)     updated     =  models.DateTimeField(auto_now = True)     timestamp   =  models.DateTimeField(auto_now_add = True)     active      = models.BooleanField(default = True)      objects = UrlManager()      def save(self, *args, **kwargs):          if self.shortcode is  None or self.shortcode == "":             self.shortcode = create_shortcode(self)          super(URL, self).save(*args, **kwargs)      def __str__(self):         return str(self.url)      def __unicode__(self):         return str(self.url)      # class Meta:     #   ordering = '-id' 

Can someone explain the the reason of error to me and how to solve it? I'm open to provide more informations IF required.

Answer 1

Another way this error can occur is when you reload the module with the class in a Jupiter notebook.

Easy solution is to restart the kernel.

http://thomas-cokelaer.info/blog/2011/09/382/

Check out @Mike W's answer for more detail.

Answer 2

You should call super using the UrlManager class as first argument not the URL model. super cannot called be with an unrelated class/type:

From the docs,

super(type[, object-or-type]): Return a proxy object that delegates method calls to a parent or sibling class of type.

So you cannot do:

>>> class D: ...    pass ...  >>> class C: ...    def __init__(self): ...        super(D, self).__init__() ...  >>> C() Traceback (most recent call last):   File "<stdin>", line 1, in <module>   File "<stdin>", line 3, in __init__ TypeError: super(type, obj): obj must be an instance or subtype of type 

You should do:

qs_main = super(UrlManager, self).all(*args, **kwargs) 

Or in Python 3:

qs_main = super().all(*args, **kwargs)