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To: python-list@python.org
From: Peter Otten <__peter__@web.de>
Subject: Re: Basic misunderstanding on object creation
Date: Wed, 13 May 2015 16:55:31 +0200
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References: <25ba3a96-21ee-4a83-b7c1-8ac60508d30c@googlegroups.com> <mivmr0$1mb$1@r01.glglgl.de> <06301cb8-39a6-410b-9fad-afe6bd4d3217@googlegroups.com> <e09c84fe-609c-4794-a117-fc72c8ccf3ad@googlegroups.com>
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Xref: csiph.com comp.lang.python:90559

andrew cooke wrote:

>> But then nothing will be passed to __init__ on the subclass.
>> 
>> Andrew
> 
>>>> class Foo:
> ...     def __new__(cls, *args, **kargs):
> ...         print('new', args, kargs)
> ...         super().__new__(cls)
> ...
>>>> class Bar(Foo):
> ...     def __init__(self, a):
> ...         print('init', a)
> ...
>>>> Bar(1)
> new (1,) {}
> 
> no "init" is printed.


That's because yor __new__() returns None. Try

$ cat new_demo.py
class Foo:
    def __new__(cls, *args, **kargs):
        print('new', cls, args, kargs)
        return super().__new__(cls)

class Bar(Foo):
    def __init__(self, a):
        print('init', a)

bar = Bar(42)
$ python3 new_demo.py 
new <class '__main__.Bar'> (42,) {}
init 42
$