Instances of many builtin types are not weak referencable.

>>> weakref.ref(0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: cannot create weak reference to 'int' object
>>> weakref.ref([])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: cannot create weak reference to 'list' object