singleton.py

# Singleton.py

class SingletonClass:
    # __init__ will be triggered if the obj returned from __new__ is instance of __class__
# thus __init__ should also check
    _ = None
    def __new__(cls, *a, **kw):
        if cls._:
            return cls._
        else:
            print("new")
            return super().__new__(cls)

    def __init__(self, *a, **kw):
        if __class__._:
            return 
        print("init")
        __class__._ = self

for i in range(5):
    print(SingletonClass())


class SingletonMetaclass(type):
    _ = None
    def __call__(cls, *a, **kw):
        if __class__._:
            return __class__._
        else:
            obj = super().__call__(*a, **kw)
            __class__._ = obj
            return obj

class C(metaclass=SingletonMetaclass):
    def __new__(cls, *a, **kw):
        print("new")
        return super().__new__(cls)
    
    def __init__(self, *a, **kw):
        print("init")

for i in range(5):
    print(C())
    
"""
new
init
<__main__.SingletonClass object at 0xffeda910>
<__main__.SingletonClass object at 0xffeda910>
<__main__.SingletonClass object at 0xffeda910>
<__main__.SingletonClass object at 0xffeda910>
<__main__.SingletonClass object at 0xffeda910>
new
init
<__main__.C object at 0xffedaa50>
<__main__.C object at 0xffedaa50>
<__main__.C object at 0xffedaa50>
<__main__.C object at 0xffedaa50>
<__main__.C object at 0xffedaa50>
"""