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devel / comp.lang.python / Re: Using __new__

Re: Using __new__

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Subject: Re: Using __new__
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by: MRAB - Sat, 17 Feb 2024 23:08 UTC

On 2024-02-17 22:35, Jonathan Gossage via Python-list wrote:
> I am attempting to use the __new__ method in the following code:
> class SingletonExample(object):
>
> _instance = None
>
> def __new__(cls, **kwargs):
> if cls._instance is None:
> cls._instance = super().__new__(cls, **kwargs)
> return cls._instance
>
> def __init__(self, **kwargs) -> None:
> our_attributes = ('h', 'x')
> if kwargs is not None:
> for k, v in kwargs.items():
> if k in our_attributes:
> setattr(self, k, v)
>
> a = SingletonExample(h=1)
>
> and I get the following result:
>
> (PRV) jonathan@jfgdev:/PR$ python -m Library.Testing.test2
> Traceback (most recent call last):
> File "<frozen runpy>", line 198, in _run_module_as_main
> File "<frozen runpy>", line 88, in _run_code
> File "/mnt/ProgrammingRenaissance/Library/Testing/test2.py", line 16, in
> <module>
> a = SingletonExample(h=1)
> ^^^^^^^^^^^^^^^^^^^^^
> File "/mnt/ProgrammingRenaissance/Library/Testing/test2.py", line 6, in
> __new__
> cls._instance = super().__new__(cls, **kwargs)
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> TypeError: object.__new__() takes exactly one argument (the type to
> instantiate)
>
> I am quite puzzled as it looks as if this code will not work if the
> super-class is 'object'. Any suggestions on how to proceed?
>
Don't pass kwargs to object.__new__ because it doesn't expect it.

Incidentally, kwargs will never be None, and there's no point in giving
a return type for __init__ because it can only ever return None.