Why cannot I use __slots__ and weakrefs together?

[Christian Heimes]

On 2018-05-26 11:17, Steven D'Aprano wrote:
> Here is my code:
> 
> 
> 
> ---- cut here %< ----
> 
> import weakref
> d = weakref.WeakValueDictionary()
> 
> class Spam:
>     pass
> 
> class Eggs:
>     __slots__ = ['spanish_inquisition']
> 
> d['a'] = Spam()  # Okay.
> d['b'] = Eggs()  # Nobody will expect what happens next!
> 
> ---- cut here %< ----
> 
> 
> and the result I get is:
> 
> 
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
>   File "/usr/local/lib/python3.5/weakref.py", line 158, in __setitem__
>     self.data[key] = KeyedRef(value, self._remove, key)
>   File "/usr/local/lib/python3.5/weakref.py", line 306, in __new__
>     self = ref.__new__(type, ob, callback)
> TypeError: cannot create weak reference to 'Eggs' object
> 
> 
> 
> Why does weakref hate my Eggs class?

Weakref needs some place to store reference information. It works if you
add "__weakref__" to your slots:

class Eggs:
     __slots__ = ['spanish_inquisition', '__weakref__']

Christian