CODEWITHSUNDEEP
pythoninheritancepython-2.xsuper
Geo
Geo

Reputation: 96767

Python super() raises TypeError

In Python 2.5, the following code raises a TypeError:

>>> class X:
      def a(self):
        print "a"

>>> class Y(X):
      def a(self):
        super(Y,self).a()
        print "b"

>>> c = Y()
>>> c.a()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in a
TypeError: super() argument 1 must be type, not classobj

If I replace the class X with class X(object), it will work. What's the explanation for this?

Upvotes: 109

Views: 47339

Answers (4)

Serafina Brocious
Serafina Brocious

Reputation: 30609

The reason is that super() only operates on new-style classes, which in the 2.x series means extending from object:

>>> class X(object):
        def a(self):
            print 'a'

>>> class Y(X):
        def a(self):
            super(Y, self).a()
            print 'b'

>>> c = Y()
>>> c.a()
a
b

Upvotes: 133

Abhi Tk
Abhi Tk

Reputation: 81

In case none of the above answers mentioned it clearly. Your parent class needs to inherit from "object", which would essentially turn it into a new style class. 

# python 3.x:
class ClassName(object): # This is a new style class
    pass

class ClassName: # This is also a new style class ( implicit inheritance from object )
    pass

# Python 2.x:
class ClassName(object): # This is a new style class
    pass

class ClassName:         # This is a old style class
    pass

Upvotes: 3

weberc2
weberc2

Reputation: 7898

I tried the various X.a() methods; however, they seem to require an instance of X in order to perform a(), so I did X().a(self), which seems more complete than the previous answers, at least for the applications I've encountered. It doesn't seem to be a good way of handling the problem as there is unnecessary construction and destruction, but it works fine.

My specific application was Python's cmd.Cmd module, which is evidently not a NewStyle object for some reason.

Final Result:

X().a(self)

Upvotes: 1

bobince
bobince

Reputation: 536339

In addition, don't use super() unless you have to. It's not the general-purpose "right thing" to do with new-style classes that you might suspect.

There are times when you're expecting multiple inheritance and you might possibly want it, but until you know the hairy details of the MRO, best leave it alone and stick to:

 X.a(self)

Upvotes: 15

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