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OOP

call super constructor

class A(object):
    def __init__(self):
        print("world")
 
class B(A):
    def __init__(self):
        print("hello")
        super().__init__()

singletons

class MySingleton:
    instance = None
 
    def __new__(cls, *args, **kwargs):
        if not isinstance(cls.instance, cls):
            cls.instance = object.__new__(cls)
        return cls.instance
class Singleton(type):
    _instances = {}
    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super (Singleton,cls).__call__(*args, **kwargs)
        return cls._instances[cls]
 
class SerialNumber(metaclass=Singleton):