python override __new__ cannot send arguments to __init__

Question

I have this very simple program that has a singleton implemented as a base class that simply defines a new that is used implement the "single-ness". However I can no longer send arguments to the init method - when I do I get a "TypeError: object() takes no parameters" from the singleton new super call:

class Singleton(object):
    _instances = {}

    def __new__(cls, *args, **kwargs):
        print(args, kwargs)
        if cls._instances.get(cls, None) is None:
            cls._instances[cls] = super(Singleton, cls).__new__(cls, *args, **kwargs)
        return Singleton._instances[cls]


class OneOfAKind(Singleton):

    def __init__(self):
        print('--> OneOfAKind __init__')
        Singleton.__init__(self)


class OneOfAKind2(Singleton):

    def __init__(self, onearg):
        print('--> OneOfAKind2 __init__')
        Singleton.__init__(self)
        self._onearg = onearg


x = OneOfAKind()
y = OneOfAKind()
print(x == y)
X = OneOfAKind2('testing')

The output is:

() {}
Traceback (most recent call last):
  File "./mytest.py", line 29, in <module>
    x = OneOfAKind()
  File "./mytest.py", line 10, in __new__
    cls._instances[cls] = super(Singleton, cls).__new__(cls, (), {})
TypeError: object() takes no parameters

Answer 1

Right, because object which is what you are inheriting from and calling from super does not take any args:

In [54]: object('foo', 'bar')
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-54-f03d0963548d> in <module>()
----> 1 object('foo', 'bar')

TypeError: object() takes no parameters

if you want to do something like this, I recommend a metaclass instead of subclassing, and instead of overriding __new__ the metaclass overrides __call__ for object creation:

import six ## used for compatibility between py2 and py3

class Singleton(type):
    _instances = {}

    def __call__(cls, *args, **kwargs):
        if Singleton._instances.get(cls, None) is None:
            Singleton._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
        return Singleton._instances[cls]

@six.add_metaclass(Singleton)
class OneOfAKind(object):

    def __init__(self):
        print('--> OneOfAKind __init__')

@six.add_metaclass(Singleton)
class OneOfAKind2(object):

    def __init__(self, onearg):
        print('--> OneOfAKind2 __init__')
        self._onearg = onearg

Then:

In [64]: OneOfAKind() == OneOfAKind()
--> OneOfAKind __init__
Out[64]: True

In [65]: OneOfAKind() == OneOfAKind()
Out[65]: True

In [66]: OneOfAKind() == OneOfAKind2('foo')
Out[66]: False