[Python-3000] Pickle 3 and keyword arguments in __new__ after *

[Amaury Forgeot d'Arc]

Hello,

Zaur Shibzoukhov wrote:
> This is a case:
>
> class C(object):
>    def __new__(cls, *, b):
>        inst = super().__new__(cls)
>        inst.b = b
>        return inst
>
>>>> c = C(b=17)
>>>> image = pickle.dumps(c, protocol=3)
>>>> c = pickle.loads(image)
> Traceback (most recent call last):
>  File "test_new.py", line 17, in <module>
>    c = pickle.loads(image)
>  File "D:\Python30\lib\pickle.py", line 1329, in loads
>    return Unpickler(file, encoding=encoding, errors=errors).load()
> TypeError: __new__() needs keyword-only argument b
>
> Do we need to improve pickle protocol in order to allow instance
> creation functions get keyword arguments too?

Note that you have a similar error even when the argument is a regular one:

class C(object):
   def __new__(cls, b):
       inst = super().__new__(cls)
       inst.b = b
       return inst

>>> c = C(b=17)
>>> image = pickle.dumps(c, protocol=3)
>>> c = pickle.loads(image)
Traceback (most recent call last):
  File "c:\temp\t.py", line 11, in <module>
    c = pickle.loads(image)
  File "c:\Python30\lib\pickle.py", line 1329, in loads
    return Unpickler(file, encoding=encoding, errors=errors).load()
TypeError: __new__() takes exactly 2 positional arguments (1 given)

A __getnewargs__ method is needed. In this case, it should return (self.b,).

Now, I would like to translate your question to "Is there an
equivalent of __getnewargs__ that can pass keyword arguments to
__new__?". The only solution I could find involves a global factory
function that calls the constructor, but does not use keyword
arguments:

class C(object):
    def __new__(cls, *, b):
        inst = super(C, cls).__new__(cls)
        inst.b = b
        return inst
    def __reduce__(self):
        return build_C, (self.b,)
def build_C(b):
    return C(b=b)

Is there a more object-oriented way?

-- 
Amaury Forgeot d'Arc