__init__ is not invoked
Rhodri James
rhodri at kynesim.co.uk
Thu Sep 26 08:36:54 EDT 2019
On 26/09/2019 13:20, ast wrote:
> Hello
>
> In the following code found here:
> https://www.pythonsheets.com/notes/python-object.html
>
> __init__ is not invoked when we create an object
> with "o = ClassB("Hello")". I don't understand why.
> I know the correct way to define __new__ is to write
> "return object.__new__(cls, arg)" and not "return object"
>
>
> >>> class ClassB(object):
> ... def __new__(cls, arg):
> ... print('__new__ ' + arg)
> ... return object
> ... def __init__(self, arg):
> ... print('__init__ ' + arg)
> ...
>
> >>> o = ClassB("Hello")
> __new__ Hello
>
>
> Normaly, when running "o = ClassB("Hello")", we first run
> __call__ from type of ClassB which is type. This __call__
> method is supposed to launch __new__ from ClassB and then
> __init__ from classB too. The output of __new__ is mapped
> to self parameter of __init__.
> But it seems things don't work like that here. Why ?
One more command to the REPL helps make things clearer:
>>> o
<class 'object'>
I would venture that o.__init__() is called, but object's __init__
doesn't do anything. On top of that, the language definition states
that "The return value of __new__() should be the new object instance
(usually an instance of cls)". You have returned the class object, not
an instance of anything, so on the whole it's lucky that
object.__init__() does nothing!
--
Rhodri James *-* Kynesim Ltd
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