Modifying class variable before __new__

Question

我想修改类的类变量

bar

，以便将A对象的创建用

bar2

而不是

bar1执行

class A(object):
    def __new__(cls, foo):
        obj = super(A, cls).__new__(cls)
        return obj


class B(A):

    bar = "bar1"

    def __new__(cls, foo=bar):
        print("actual",foo)
        rm = super().__new__(cls,foo=foo)
        return rm

if __name__ == '__main__':

    B.bar = 'bar2'
    b = B()
    print(b.bar)

when I run the code I get:

actual bar1
bar2

whereas I'm expecting:

actual bar2
bar2

Answer 1

Just use the normal

__init__

 approach instead.您可以转发您需要的任何默认论点：

class A:
    def __init__(self, *args, **kwargs):
        print(f"A.__init__ got bar={kwargs.get('bar')}")

class B(A):
    bar = "bar1"

    def __init__(self, *args, **kwargs):
        if 'bar' not in kwargs:
            kwargs['bar'] = B.bar
        super().__init__(self, *args, **kwargs)

print(B().bar, B(bar="lol").bar)