Singleton example

Use metaclass

class Singleton(type):
    _instances = {}

    def __call__(cls, *args, **kwargs):
        print("singleton called")
        if cls not in cls._instances:
            print("singleton cls not in _instances")
            cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
        return cls._instances[cls]

class Inference(metaclass=Singleton):

    def __init__(self):
        print("inference init called")
        super().__init__()
        self.model = None

# Example
a = Inference()
"""Output
singleton called
singleton cls not in _instances
inference init called
"""

b = Inference()
# singleton called

Use _ _ new _ _

class InferenceHelper(object):

    _instance = None

    def __new__(cls, *args, **kwargs):
        """A singleton behavior
        Returns:
            InferenceHelper: The instance of InferenceHelper
        """
        if cls._instance is None:
            logger.info('Initialize a new instance of InferenceHelper.')
            cls._instance = super(InferenceHelper, cls).__new__(cls)
            cls._instance._instace_init(*args, **kwargs)
        elif cls._instance._any_new_config(**kwargs):
            # Only do it when there is different config(s)
            cls._instance._instace_init(*args, **kwargs)
        return cls._instance

Use self-defined get_instance

class InferenceHelper:

    __instance = None

    @staticmethod
    def get_instance(*args, **kwargs):
        """A singleton behavior
        Returns:
            InferenceHelper: The instance of InferenceHelper
        """
        if InferenceHelper.__instance is None:
            logger.info('Initialize a new instance of InferenceHelper.')
            InferenceHelper(*args, **kwargs)

        return InferenceHelper.__instance

        def __init__(self, *args,  **kwargs):
                if InferenceHelper.__instance is not None:
            raise Exception("You cannot manually create an instance of InferenceHelper! Call the get_instance instead")
        else:
                        # Do init job here....
                        pass

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