Multiple inheritance : waht does this error mean ?

[jnair]

I am using Python 2.4.3

class K(object,list):

...: pass
...:
------------------------------------------------------------
Traceback (most recent call last):
File "<console>", line 1, in ?
TypeError: Error when calling the metaclass bases
Cannot create a consistent method resolution
order (MRO) for bases object, list
Regards
Jitendra Nair

[Ben Finney]

jn***@ensim.com writes:

class K(object,list):

...: pass
...:
------------------------------------------------------------
Traceback (most recent call last):
File "<console>", line 1, in ?
TypeError: Error when calling the metaclass bases
Cannot create a consistent method resolution
order (MRO) for bases object, list

The class 'object' is already the base class of 'list', and of every
other basic type.

Python is saying that you need to decide what you want; inheritance
from 'object', or inheritance from something that already inherits
from 'object'.

--
\ "People come up to me and say, 'Emo, do people really come up |
`\ to you?'" -- Emo Philips |
_o__) |
Ben Finney

[John Machin]

Question for you: what do you think that "class K(object,list):"
means?? What are you trying to achieve? What do you plan to do with K?
If you want it to be a subclass of list, all you have to do is "class
K(list):".

[Peter Otten]

jn***@ensim.com wrote:

I am using Python 2.4.3

class K(object,list): ...: pass
...:
------------------------------------------------------------
Traceback (most recent call last):
File "<console>", line 1, in ?
TypeError: Error when calling the metaclass bases
Cannot create a consistent method resolution
order (MRO) for bases object, list

class K(object, list): pass

specifies that a method should be looked up first in object, then in list.
As list inherits from object, its method resolution order

list.__mro__ (<type 'list'>, <type 'object'>)

specifies that a method should be looked up first in list, then in object.
Python refuses to resolve the conflict for you, but you can do it manually:
class K2(list, object): pass .... K2.__mro__ (<class '__main__.K2'>, <type 'list'>, <type 'object'>)

Now K2, list, and object can agree to look for a method in K2, then list,
then object. However, you get the same effect by just inheriting from list:
class K3(list): pass .... K3.__mro__

(<class '__main__.K3'>, <type 'list'>, <type 'object'>)

Peter

[Michele Simionato]

jn***@ensim.com wrote:

I am using Python 2.4.3

class K(object,list):

...: pass
...:
------------------------------------------------------------
Traceback (most recent call last):
File "<console>", line 1, in ?
TypeError: Error when calling the metaclass bases
Cannot create a consistent method resolution
order (MRO) for bases object, list

It is explained here:

http://www.python.org/download/releases/2.3/mro/

Michele Simionato