Multiple inheritance : waht does this error mean ?

[jnair@ensim.com]

I am using Python 2.4.3
[color=blue][color=green][color=darkred]
>>>class K(object,list):[/color][/color][/color]
...: 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]

Re: Multiple inheritance : waht does this error mean ?

jnair@ensim.com writes:
[color=blue][color=green][color=darkred]
> >>>class K(object,list):[/color][/color]
> ...: 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[/color]

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]

Re: Multiple inheritance : waht does this error mean ?

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]

Re: Multiple inheritance : waht does this error mean ?

jnair@ensim.com wrote:
[color=blue]
> I am using Python 2.4.3
>[color=green][color=darkred]
>>>>class K(object,list):[/color][/color]
> ...: 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[/color]

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
[color=blue][color=green][color=darkred]
>>> list.__mro__[/color][/color][/color]
(<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:
[color=blue][color=green][color=darkred]
>>> class K2(list, object): pass[/color][/color][/color]
....[color=blue][color=green][color=darkred]
>>> K2.__mro__[/color][/color][/color]
(<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:
[color=blue][color=green][color=darkred]
>>> class K3(list): pass[/color][/color][/color]
....[color=blue][color=green][color=darkred]
>>> K3.__mro__[/color][/color][/color]
(<class '__main__.K3'>, <type 'list'>, <type 'object'>)

Peter

[Michele Simionato]

Re: Multiple inheritance : waht does this error mean ?

jnair@ensim.com wrote:[color=blue]
> I am using Python 2.4.3
>[color=green][color=darkred]
> >>>class K(object,list):[/color][/color]
> ...: 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[/color]

It is explained here:

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

Michele Simionato