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

Question

The following use of super() raises a TypeError: why?

>>> from  HTMLParser import HTMLParser >>> class TextParser(HTMLParser): ...     def __init__(self): ...         super(TextParser, self).__init__() ...         self.all_data = [] ...          >>> 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:

>>> 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).

Answer 1

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

>>> class OldStyle: pass >>> instance = OldStyle() >>> issubclass(instance.__class__, object) False 

and not (as in the question):

>>> isinstance(instance, object) True 

For classes, the correct "is this a new-style class" test is:

>>> issubclass(OldStyle, object)  # OldStyle is not a new-style class False >>> 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:

>>> type(OldStyle)  # OldStyle creates objects but is not itself a type classobj >>> isinstance(OldStyle, type) False >>> 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).

Answer 2

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 = []