为什么我的Python类没有调用__new__？

Question

I have a class defined like so:

class Client():
    def __new__(cls):
        print "NEW"
        return cls

    def __init__(self):
        print "INIT"

When I use it, I get the following output:

cl = Client()
# INIT

__new__ is not being called. Why?

Answer 1

Having read your answer, I improve it with

class Client(object):
    def __new__(cls):
        print "NEW"
        return super(Client, cls).__new__(cls)
    def __init__(self):
        print "INIT"

so that c = Client() outputs

NEW
INIT

as intended.

Answer 2

Classes must inherit explicitly from object in order for __new__ to be called. Redefine Client so instead it looks like:

class Client(object):
    def __new__(cls):
        print "NEW"
        return cls

    def __init__(self):
        print "INIT"

__new__ will now be called when used like:

cl = Client()
# NEW

Note that __init__ will never be called in this situation, since __new__ does not invoke the superclass's __new__ as it's return value.