Memoized objects still have their __init__() invoked? - python-3.x

Question

So I am creating a memoized class, and have been observing a strange behavior.
Here's the snippet of the code:
class SomeClass(object):
_Memoized = {}
def __new__(cls, id: str, *args, **kwargs):
if id not in cls._Memoized:
print('New Instance')
cls._Memoized[id] = super(SomeClass, cls).__new__(cls, *args, **kwargs)
else:
print('Existing Instance')
return cls._Memoized[id]
def __init__(self, id: str):
print('Running init')
self.id = id
def test():
w1 = SomeClass(id='w1')
wx = SomeClass(id='w1')
print(id(w1), id(wx), id(w1) == id(wx))
test()
Running the above code results in:
New Instance
Running init
Existing Instance
Running init <===-------------------???
140008534476784 140008534476784 True
My questions: During the second invocation of SomeClass(), why does it execute the __init__ constructor? Wasn't the __init__ constructor only invoked on instantiating? Is there a way to prevent the __init__ from being invoked?

Answer 1

The purpose of __new__ is to create a new instance, which is why Python calls __init__ on it. You might instead override __call__ on a metaclass to avoid creating a new instance.
class MemoMeta(type):
def __init__(self, name, bases, namespace):
super().__init__(name, bases, namespace)
self.cache = {}
def __call__(self, id_, *args, **kwargs):
if id_ not in self.cache:
print('New Instance')
self.cache[id_] = super().__call__(id_, *args, **kwargs)
else:
print('Existing Instance')
return self.cache[id_]
class SomeClass(metaclass=MemoMeta):
def __init__(self, id_, *args, **kwargs):
print('Running init')
self.id = id_
def test():
w1 = SomeClass(id_='w1')
wx = SomeClass(id_='w1')
print(id(w1), id(wx), id(w1) == id(wx))
test()