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__slots__ and multiple inheritance

Carl Banks imbosol at vt.edu
Sat Dec 14 15:53:45 EST 2002 
Dave Reed wrote:
> 
> __slots__ doesn't seem to work as I expect when I use mixins/multiple
> inheritance:
> 
> I've got a BaseSQL class (see code at bottom of message) in which I
> define __slots__ (I started using it so pychecker didn't complain
> about me using the instance variables from the other base class). I
> left out "values" from __slots__, but it still lets me assign to
> self.values.
> 
> Also, If I put the same __slots__ in the StudentSQL class, I get the
> error:
> 
>    class Student(BaseSQL, StudentSQL):
> TypeError: multiple bases have instance lay-out conflict
> 
> Can anyone explain to me how this should work?

It doesn't work at all.  It seems kind of stupid at first, but you
can't inherit from two classes with slots (well, unless those classes'
slots are all inherited from a common base class).

The reason for this is that slots have fixed positions in the object's
low-level structure.  One base class puts its slots at position x in
the structure; another base class also puts slots at position x.  You
can't multiply inherit from both of them because of the clash.


-- 
CARL BANKS
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