TypeError:object()不带参数 - 但仅限于Python 3

Question

我正在将一些代码从Python 2迁移到Python 3,并且我得到了不同的行为.查看"改变了什么"的列表并未指出任何相关的差异,但可能我错过了一个重要的差异.

我尽可能地简化了我的代码以获得这个"最小错误程序":

def decorator(Type):
    """ This is a class decorator. It replaces a class with a subclass which
    *should be* equivalent.

    The result works on Python 2.7 but not on Python 3.4. """

    class FactorySubclass(Type):
        """ This subclasses from the provided type, and overrides the __new__
            and __init__ methods, but replaces them with exact equivalents,
            so I can't see how this has any effect. """

        def __new__(cls, *args, **kwargs):
            # Simplified this code to do basically nothing.
            # If this line is removed, it works on both versions.
            return Type.__new__(cls, *args, **kwargs)

        def __init__(self, *args, **kwargs):
            # Simplified this code to do basically nothing.
            Type.__init__(self, *args, **kwargs)

    return FactorySubclass


@decorator
class ExampleClass(object):
    def __init__(self, param=3):
        print("Constructed example instance")


ec = ExampleClass(param=5)

此代码运行,并Constructed example instance在Python 2.7中打印.此代码失败,并在Python 3.4中转储堆栈跟踪.

Traceback (most recent call last):
  File "mfp.py", line 31, in <module>
    ec = ExampleClass(param=5)
  File "mfp.py", line 16, in __new__
    return Type.__new__(cls, *args, **kwargs)
TypeError: object() takes no parameters

通常这个错误意味着某人拼写错误__init__(因此构造函数参数绕过相关类并被赋予无object参数构造函数,但这似乎不是这种情况.

哦,作为事后的想法,我确认,是的,paramPython 2.7中的值为5.

2to3 给它一个清洁的健康状况.

请给我一个指向Python 3中的更改的指针,该更改将使此代码无效,因此我可能会阅读更多相关内容.

Answer 1

你的问题里已经有了答案：

通常，此错误意味着 [...] 构造函数参数被赋予对象的无参数构造函数 [...]

要修复此问题，请将装饰器更改为仅添加__init__and__new__如果传入的Type方法具有这些方法。