python - __new__ 在对象创建中没有被调用

Question

案例1:

class Person:
    def __new__(cls, *args, **kwargs):
        print "called"
        return super(Person, cls).__new__(cls, *args, **kwargs)

p=Person()

案例2:

class Person(object):
    def __new__(cls, *args, **kwargs):
        print "called"
        return super(Person, cls).__new__(cls, *args, **kwargs)

p=Person()

在第一种情况下，不会调用 __new__() 方法，但在第二种情况下会调用。

如果没有被调用，那么 Person 对象是如何创建的？

Answer 1

我猜这与 new and old style classes 有关。在Python2中:

Old-style classes don't actually have a __new__ method because for them __init__ is the constructor, so basically if we would have:

class A:

    def __new__(cls):
        print "A.__new__ is called"  # -> this is never called

A()

the body of __new__ will never be executed in this case because it is not the purpose for old-style classes.

在 Python3 中，行为是相同的，无论您是否显式继承对象:

class Person1:
    def __new__(cls, *args, **kwargs):
        print("called")
        return super(Person1, cls).__new__(cls, *args, **kwargs)


class Person2(object):
    def __new__(cls, *args, **kwargs):
        print("called")
        return super(Person2, cls).__new__(cls, *args, **kwargs)


p1 = Person1()
p2 = Person2()

当从 3.x 调用时，这些应该打印“called”两次。