EOL EOL - 2 months ago 23
Python Question

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

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 ! Why?, 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: http://stackoverflow.com/revisions/2655651/3).

EOL EOL
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

>>> 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: http://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).

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