super() fails with error: TypeError "argument 1 must be type, not classobj" when parent does not inherit from object
I get some error that I can't figure out. Any clue what is wrong with my sample code?
class B:
def meth(self, arg):
print arg
class C(B):
def meth(self, arg):
super(C, self).meth(arg)
print C().meth(1)
I got the sample test code from help of 'super' built-in method.
Here is the error:
Traceback (most recent call last):
File "./test.py", line 10, in ?
print C().meth(1)
File "./test.py", line 8, in meth
super(C, self).meth(arg)
TypeError: super() argument 1 must be type, not classobj
FYI, here is the help(super) from python itself:
Help on class super in module __builtin__:
class super(object)
| super(type) -> unbound super object
| super(type, obj) -> bound super object; requires isinstance(obj, type)
| super(type, type2) -> bound super object; requires issubclass(type2, type)
| Typical use to call a cooperative superclass method:
| class C(B):
| def meth(self, arg):
| super(C, self).meth(arg)
|
Your problem is that class B is not declared as a "new-style" class. Change it like so:
class B(object):
and it will work.
super() and all subclass/superclass stuff only works with new-style classes. I recommend you get in the habit of always typing that (object) on any class definition to make sure it is a new-style class.
Old-style classes (also known as "classic" classes) are always of type classobj; new-style classes are of type type. This is why you got the error message you saw:
TypeError: super() argument 1 must be type, not classobj
Try this to see for yourself:
class OldStyle:
pass
class NewStyle(object):
pass
print type(OldStyle) # prints: <type 'classobj'>
print type(NewStyle) # prints <type 'type'>
Note that in Python 3.x, all classes are new-style. You can still use the syntax from the old-style classes but you get a new-style class. So, in Python 3.x you won't have this problem.
Also, if you can't change class B, you can fix the error by using multiple inheritance.
class B:
def meth(self, arg):
print arg
class C(B, object):
def meth(self, arg):
super(C, self).meth(arg)
print C().meth(1)
type) while still subclassing an "old-style" class (whose class object is of type classobj). super() works with new-style classes but not with old-style classes.
If the python version is 3.X, it's okay.
I think your python version is 2.X, the super would work when adding this code
__metaclass__ = type
so the code is
__metaclass__ = type
class B:
def meth(self, arg):
print arg
class C(B):
def meth(self, arg):
super(C, self).meth(arg)
print C().meth(1)
I was also faced by the posted issue when I used python 2.7. It is working very fine with python 3.4
To make it work in python 2.7 I have added the __metaclass__ = type attribute at the top of my program and it worked.
__metaclass__ : It eases the transition from old-style classes and new-style classes.
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super(); class A must be made to work with an "old-style" class, and possibly the best way to do that would be to make class A be itself an "old-style" class. Of course I recommend just upgrading your whole program to run in Python 3.x, so that all classes will be new-style no matter what you do; if that option is available it's the best option.class B(object):. I'm getting this error due to using@mock.patch('module.B', autospec=B)just before my test case. Any thoughts on how to fix this?