>>> import weakref
>>> class A: pass
... 
>>> a = A()
>>> d = weakref.WeakValueDictionary(spam=a)
>>> list(d.items())
[('spam', <__main__.A object at 0xb6f3f88c>)]
>>> weakref.WeakValueDictionary(self=a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __init__() got multiple values for argument 'self'
>>> weakref.WeakValueDictionary(dict=a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/serhiy/py/cpython/Lib/weakref.py", line 114, in __init__
    self.update(*args, **kw)
  File "/home/serhiy/py/cpython/Lib/weakref.py", line 261, in update
    dict = type({})(dict)
TypeError: 'A' object is not iterable
>>> d = weakref.WeakValueDictionary()
>>> d.update(spam=a)
>>> list(d.items())
[('spam', <__main__.A object at 0xb6f3f88c>)]
>>> d.update(self=a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: update() got multiple values for argument 'self'
>>> d.update(dict=a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/serhiy/py/cpython/Lib/weakref.py", line 261, in update
    dict = type({})(dict)
TypeError: 'A' object is not iterable