为什么我不能在python3中子类化元组？

Question

让我们先说这个问题,你应该使用__new__而不是__init__用于子类化不可变对象.

话虽如此,让我们看看以下代码:

class MyTuple(tuple):
    def __init__(self, *args):
        super(MyTuple, self).__init__(*args)

mytuple = MyTuple([1,2,3])

这适用于python2,但在python3中,我得到:

Traceback (most recent call last):
  File "tmp.py", line 5, in <module>
    mytuple = MyTuple([1,2,3])
  File "tmp.py", line 3, in __init__
    super(MyTuple, self).__init__(*args)
TypeError: object.__init__() takes no parameters

为什么会这样？python3有什么变化？

Answer 1

当重写两者时,Python 3改变了对参数的反应object.__new__和object.__init__反应.如果一个类重写(或继承覆盖的方法)object.__init__和object.__new__,object.__init__并且object.__new__如果它们收到任何多余的参数将抛出异常.在Python 2中,这将给出DeprecationWarning(默认情况下被禁止).

tuple没有自己的__init__.它继承了object.__init__,所以你实际上传递了一堆论据object.__init__是object.__init__不走.Python 2给你一个(压制)警告,而Python 3正在使它成为一个错误.

代码有一个注释,可以很好地解释object.__init__和object.__new__微妙处理额外的参数:

/* You may wonder why object.__new__() only complains about arguments
   when object.__init__() is not overridden, and vice versa.

   Consider the use cases:

   1. When neither is overridden, we want to hear complaints about
      excess (i.e., any) arguments, since their presence could
      indicate there's a bug.

   2. When defining an Immutable type, we are likely to override only
      __new__(), since __init__() is called too late to initialize an
      Immutable object.  Since __new__() defines the signature for the
      type, it would be a pain to have to override __init__() just to
      stop it from complaining about excess arguments.

   3. When defining a Mutable type, we are likely to override only
      __init__().  So here the converse reasoning applies: we don't
      want to have to override __new__() just to stop it from
      complaining.

   4. When __init__() is overridden, and the subclass __init__() calls
      object.__init__(), the latter should complain about excess
      arguments; ditto for __new__().

   Use cases 2 and 3 make it unattractive to unconditionally check for
   excess arguments.  The best solution that addresses all four use
   cases is as follows: __init__() complains about excess arguments
   unless __new__() is overridden and __init__() is not overridden
   (IOW, if __init__() is overridden or __new__() is not overridden);
   symmetrically, __new__() complains about excess arguments unless
   __init__() is overridden and __new__() is not overridden
   (IOW, if __new__() is overridden or __init__() is not overridden).

   However, for backwards compatibility, this breaks too much code.
   Therefore, in 2.6, we'll *warn* about excess arguments when both
   methods are overridden; for all other cases we'll use the above
   rules.

*/