[Zope] Re: Error:unbound method __init__() must be called with instance as first argument

[Casey Duncan]

On Fri, 9 Apr 2004 16:28:38 +0400
"Serg" <sutni at yandex.ru> wrote:

> --- file A.py ---
> class CA:
>     "class for my needs"
>     def __init__(self, a=0):
>         "init"
>         self.__a=a
> 
> --- file B.py ---
> from A import CA
> from OFS import SimpleItem
> class CB(SimpleItem.SimpleItem , CA ):
>     "class B"
>     def __init__(self, id, docN):
>         "init"
>         CA.__init__(self)
>         self.id=id
>         self.docN=docN
> 
>  On add object of CB Zope write:
>         Site Error
>         An error was encountered while publishing this resource.
>         Error Type: TypeError
>         Error Value: unbound method __init__() must be called with
>         instance
> as first argument
> 
> Where is this Error?

The error is that CB and CA are different types. CA is a classic python
class. CB is an ExtensionClass (because it subclasses SimpleItem which
subclasses ExtensionClass.Base). Python is preventing you from binding
the non-extension class CA to the CB extension class intance.

A solution is to make CA an extension class::

  import ExtensionClass

  class CA(ExtensionClass.Base):
      ...

Then delegating to CA's __init__ from CB should work fine. This is the
reason why you will often see mix-in classes in Zope inherit from
ExtensionClass.Base even though they do not explicitly use its features.

-Casey