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Dict-like types in the weakref module (WeakValueDictionary and WeakKeyDictionary) don't allow to specify key-value pair as keyword arguments if key is "self" or "dict".
>>> 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
Related issue for the collections module is issue22609. I think weakref mapping classes should be fixed in the same manner.

Here is a patch similar to patch from issue22609 which makes WeakValueDictionary constructor and update accept keyword arguments "self" and "dict".

lgtm

New changeset 8274fc521e69 by Serhiy Storchaka in branch '2.7':
Issue #22958: Constructor and update method of weakref.WeakValueDictionary
https://hg.python.org/cpython/rev/8274fc521e69
New changeset 01c79072d671 by Serhiy Storchaka in branch '3.4':
Issue #22958: Constructor and update method of weakref.WeakValueDictionary
https://hg.python.org/cpython/rev/01c79072d671
New changeset 73b6b88ac28a by Serhiy Storchaka in branch '3.5':
Issue #22958: Constructor and update method of weakref.WeakValueDictionary
https://hg.python.org/cpython/rev/73b6b88ac28a
New changeset 815bb6a2d69e by Serhiy Storchaka in branch 'default':
Issue #22958: Constructor and update method of weakref.WeakValueDictionary
https://hg.python.org/cpython/rev/815bb6a2d69e