This code:

import weakref
import attr

@attr.s(slots=True)
class Foo(object):
    x = attr.ib()

weakref.ref(Foo('x'))

produces this error:

Traceback (most recent call last):
  File "slotsref.py", line 8, in <module>
    weakref.ref(Foo('x'))
TypeError: cannot create weak reference to 'Foo' object

In the docs for __slots__, it says "Without a __weakref__ variable for each instance, classes defining __slots__ do not support weak references to its instances. If weak reference support is needed, then add '__weakref__' to the sequence of strings in the __slots__ declaration."

Adding '__weakref__' to the list isn't hard, but I wanted to see if people had any opinions about whether this should happen transparently, or if you should have to opt-in to it by saying @attr.s(weakref=True).