Using __new__

[dn]

On 18/02/24 11:35, 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 be puzzled. Read the error-message.

Change the super-call to: cls._instance = super().__new__(cls)
and happiness will follow...


That said, mystifications - not sure if this meets the/your definition* 
of "singleton", because:

- it can be aliased, eg
a = SingletonExample(h=1)
b = SingletonExample(x=2)

- when it is, the effect is an accumulation of attributes and values
a = SingletonExample(h=1)
b = SingletonExample(h=2)
print( a.__dict__, b.__dict__, )

- it can be re-created with a different value, eg
a = SingletonExample(h=1)
a = SingletonExample(h=2)

- and can be 'regenerated':
a = SingletonExample(h=1)
a = SingletonExample(x=2)

- all failures are silent


* noting "Nowadays, the Singleton pattern has become so popular that 
people may call something a singleton even if it solves just one of the 
listed problems." (https://refactoring.guru/design-patterns/singleton)


YMMV!

-- 
Regards,
=dn