super raises TypeError must be type not classobj for new-style class

Question

The following use of super   raises a TypeError  why    gt  gt  gt  from  HTMLParser import HTMLParser  gt  gt  gt  class TextParser HTMLParser           def   init   self               super TextParser  self    init                 self all data                    gt  gt  gt  TextParser         TypeError  must be type  not classobj   There is a similar question on StackOverflow  Python super   raises TypeError  where the error is explained by the fact that the user class is not a new-style class   However  the class above is a new-style class  as it inherits from object    gt  gt  gt  isinstance HTMLParser    object  True   What am I missing  How can I use super    here   Using HTMLParser   init   self  instead of super TextParser  self    init     would work  but I would like to understand the TypeError   PS  Joachim pointed out that being a new-style-class instance is not equivalent to being an object  I read the opposite many times  hence my confusion  example of new-style class instance test based on object instance test  https   stackoverflow com revisions 2655651 3

User · Answer

FWIW and though I m no Python guru I got by with this   gt  gt  gt  class TextParser HTMLParser          def handle starttag self  tag  attrs              if tag     b                  self all data append  bold              else                 self all data append  other                          gt  gt  gt  p   TextParser    gt  gt  gt  p all data       gt  gt  gt  p feed text   gt  gt  gt  print p all data         Just got me the parse results back as needed

User · Answer

the correct way to do will be as following in the old-style classes which doesn t inherit from  object   class A      def foo self           return  Hi there   class B A       def foo self  name           return A foo self    name

User · Answer

The problem is that super needs an object as an ancestor    gt  gt  gt  class oldstyle          def   init   self   self os   True   gt  gt  gt  class myclass oldstyle           def   init   self   super myclass  self    init       gt  gt  gt  myclass   TypeError  must be type  not classobj   On closer examination one finds    gt  gt  gt  type myclass  classobj   But    gt  gt  gt  class newstyle object   pass   gt  gt  gt  type newstyle  type         So the solution to your problem would be to inherit from object as well as from HTMLParser  But make sure object comes last in the classes MRO    gt  gt  gt  class myclass oldstyle  object           def   init   self   super myclass  self    init       gt  gt  gt  myclass   os True

User · Answer

Alright  it s the usual  super   cannot be used with an old-style class    However  the important point is that the correct test for  is this a new-style instance  i e  object    is   gt  gt  gt  class OldStyle  pass  gt  gt  gt  instance   OldStyle    gt  gt  gt  issubclass instance   class    object  False   and not  as in the question     gt  gt  gt  isinstance instance  object  True   For classes  the correct  is this a new-style class  test is    gt  gt  gt  issubclass OldStyle  object     OldStyle is not a new-style class False  gt  gt  gt  issubclass int  object     int is a new-style class True   The crucial point is that with old-style classes  the class of an instance and its type are distinct  Here  OldStyle     class   is OldStyle  which does not inherit from object  while type OldStyle    is the instance type  which does inherit from object   Basically  an old-style class just creates objects of type instance  whereas a new-style class creates objects whose type is the class itself   This is probably why the instance OldStyle   is an object  its type   inherits from object  the fact that  its class does not inherit from object does not count  old-style classes merely construct new objects of type instance   Partial reference  https   stackoverflow com a 9699961 42973   PS  The difference between a new-style class and an old-style one can also be seen with    gt  gt  gt  type OldStyle     OldStyle creates objects but is not itself a type classobj  gt  gt  gt  isinstance OldStyle  type  False  gt  gt  gt  type int     A new-style class is a type type    old-style classes are not types  so they cannot be the type of their instances

User · Answer

You can also use class TextParser HTMLParser  object    This makes TextParser a new-style class  and super   can be used

User · Answer

super   can be used only in the new-style classes  which means the root class needs to inherit from the  object  class   For example  the top class need to be like this   class SomeClass object       def   init   self                  not  class SomeClass        def   init   self                  So  the solution is that call the parent s init method directly  like this way   class TextParser HTMLParser       def   init   self           HTMLParser   init   self          self all data

User · Answer

If you look at the inheritance tree  in version 2 6   HTMLParser inherits from SGMLParser which inherits from ParserBase which doesn t inherits from object  I e  HTMLParser is an old-style class   About your checking with isinstance  I did a quick test in ipython    In  1   class A              pass           In  2   isinstance A  object  Out 2   True   Even if a class is old-style class  it s still an instance of object

[python] super() raises "TypeError: must be type, not classobj" for new-style class

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