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@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
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