python override __new__无法向__init__发送参数

Question

我有一个非常简单的程序，它有一个单例实现为基类，它只是定义了一个用于实现“单一”的新东西。但是我不能再向init方法发送参数 - 当我这样做时，我从单例新的超级调用中得到一个“TypeError：object（）不参数”：

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')

输出是：

() {}
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

是的，因为你继承自object并从super调用的

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

不接受任何参数：

metaclass

如果你想做这样的事情，我建议使用__new__而不是子类，而不是覆盖metaclass，__call__会覆盖

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

来创建对象：

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

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

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

然后：

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