[Ron Garret]
> Thanks for the detailed explanation. I understand now why you can't
> create weakrefs to these types. What I don't understand still is why
> you can't create weakrefs to user-defined classes that inherit from
> these types. I would think that instances of user-defined classes have
> the same header structure regardless of what they inherit from. This
> would seem to be supported by the fact that you can create weakrefs to
> instances of user-defined classes that inherit from int and float.
It is an over-statement to say that it can't be done. In fact, Michael Hudson
is already working on a patch.
It is more accurate to say that the current mechanism doesn't allow it.
Michael's solution is to build a new mechanism.
The existing mechanism has a subclass extend the superclass's structure:
[--someobj--][--subclassdata--]
^
|
|---- offset to wr table ---
The offset is fixed for the type and must be the same across instances.
This is a problem for tuples and ints because someobj is of varying length:
[--tuple header, elem0, elem1, elem2--]
[--tuple header, elem0 ]
In contrast, ints and floats floats have no problem because they are always the
same size:
[--int header, int value--]
Raymond Hettinger
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