Using __new__

[MRAB]

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 at 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.