对象在定义 __new__ 后不带参数

Question

在 python 3.3 及更高版本中，当我们覆盖 __new__() 时，我们不必将参数和关键字参数传递给 super().__new__() 或 object.__new__()。但是这个调用super().__new__()returns一个class的实例。

那么python如何将剩余的参数传递给__init__呢？

class Spam(object):
    ''' Hello I am Spam '''

    def __new__(cls, *args, **kwargs):
        print("Creating Instance")
        instance = object.__new__(cls) # Who passed *args and **kwargs to __init__?
        print(instance)
        return instance

    def __init__(self, a, b):
        print("Init Called")
        self.a = a
        self.b = b

有人可以解释一下这里发生了什么吗？

Answer 1

您将 cls 作为参数传递给了 object.__new__，因此解释器可以检查是否 instance 是 cls.

的实例

初始化器（__init__）被分配器（__new__）自动调用为[Python.Docs]: object.__new__(cls[, ...]) 状态（重点 是我的）：

If __new__() returns an instance of cls, then the new instance’s __init__() method will be invoked like __init__(self[, ...]), where self is the new instance and the remaining arguments are the same as were passed to __new__().

If __new__() does not return an instance of cls, then the new instance’s __init__() method will not be invoked.

code00.py:

#!/usr/bin/env python3

import sys


class Spam(object):
    ''' Hello I am Spam '''

    def __new__(cls, *args, **kwargs):
        print("Creating Instance")
        instance = object.__new__(cls) # Who passed *args and **kwargs to __init__?
        print(instance)
        #return instance  # If you return anything else (1, object(), or None by commenting the line) here, __init__ won't be called
        if len(sys.argv) == 1:  # DO NOT DO THIS!!! It's just for demo purposes
            return instance


    def __init__(self, a, b):
        print("Init Called")
        self.a = a
        self.b = b


def main():
    spam = Spam(1, 2)
    print(type(spam), dir(spam))


if __name__ == "__main__":
    print("Python {0:s} on {0:s}\n".format(sys.version, sys.platform))
    main()

输出:

e:\Work\Dev\Whosebug\q054511671>"e:\Work\Dev\VEnvs\py_064_03.06.08_test0\Scripts\python.exe" code00.py
Python 3.6.8 (tags/v3.6.8:3c6b436a57, Dec 24 2018, 00:16:47) [MSC v.1916 64 bit (AMD64)] on win32

Creating Instance
<__main__.Spam object at 0x000001F8E24D14E0>
Init Called
<class '__main__.Spam'> ['__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'a', 'b']

e:\Work\Dev\Whosebug\q054511671>"e:\Work\Dev\VEnvs\py_064_03.06.08_test0\Scripts\python.exe" code.py arg
Python 3.6.8 (tags/v3.6.8:3c6b436a57, Dec 24 2018, 00:16:47) [MSC v.1916 64 bit (AMD64)] on win32

Creating Instance
<__main__.Spam object at 0x0000020808F71550>
<class 'NoneType'> ['__bool__', '__class__', '__delattr__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__']

请注意，这并非特定于 Python 3（检查 [Python 2.Docs]: Data model), but rather to [Python]: New-style Classes 有关更多详细信息，您还可以检查检查[Python 2.2.Docs]: Overriding the __new__ method（Singleton class）。

Answer 2

这里重要的是初始调用，比如 spam = Spam('x', 1)。

在内部，Python 调用 __new__ 作为 class 方法 class Spam 使用传递的参数 . Spam.__new__ 实际做什么并不重要，它只是应该 return 一个对象。

它确实使用 object.__new__ 构建了一个 Spam 对象。由于创建的对象具有正确的 class，Python 使用初始参数 .

对其 调用 __init__