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from typing import ClassVar
class SingletonMeta(type):
"""
The Singleton class can be implemented in different ways in Python. Some
possible methods include: base class, decorator, metaclass. We will use the
metaclass because it is best suited for this purpose.
"""
_instances: ClassVar[dict] = {}
def __call__(cls, *args, **kwargs):
"""
Possible changes to the value of the `__init__` argument do not affect
the returned instance.
"""
if cls not in cls._instances:
instance = super().__call__(*args, **kwargs)
cls._instances[cls] = instance
return cls._instances[cls]
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