We can call methods of parenet class without initliaze it?

[Roel Schroeven]

Op 15/03/2023 om 10:57 schreef scruel tao:
> The following code won’t be allowed in Java, but in python, it works fine:
> ```python
> class A:
>      A = 3
>
>      def __init__(self):
>          print(self.A)
>
>      def p(self):
>          print(self.A)
>          self.A += 1
>
>
> class B(A):
>      def __init__(self):
>          print(2)
>          self.p()
>          super().__init__()
>
>
> B()
> ```
>
> How can I understand this? Will it be a problem?
Important: __init__ is not a constructor, like you have for example in 
C++. I don't know Java, but it seems plausible it works somewhat like 
C++ in this regard. Python does it differently: when you create an 
instance, the instance is fully created/constructed even before __init__ 
is called. __init__ is purely an initializer: you can use to initialize 
the things you want to initialize.

Back to your example: it works because A is a class-level attribute, 
which is initialized independently from __init__. If you make it an 
instance attribute, like below, things stop working:

     class A:
         def __init__(self):
             self.A = 3
             print(self.A)

         def p(self):
             print(self.A)
             self.A += 1


     class B(A):
         def __init__(self):
             print(2)
             self.p()
             super().__init__()


     B()
     print(A.A)

That fails like this:

     Traceback (most recent call last):
       File ".code.tio", line 18, in <module>
         B()
       File ".code.tio", line 14, in __init__
         self.p()
       File ".code.tio", line 7, in p
         print(self.A)
     AttributeError: 'B' object has no attribute 'A'

That's because now A is indeed initialized in A.__init__, so it doesn't 
exist before A.__init__ is called.

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