Python: Call for value inside nested class

Question

I'm trying to call for value from class B that is nested in class A and use it in class C. I'm getting AttributeError:

class A():
    class B():
        a = 1
    class C():
        b = 2
        c = B.a + b 
AttributeError: class B has no attribute 'a'

I also tried to call From 'A', Pycharm recognize it, but python still get AttributeError:

class A(object):
    class B(object):
        a = 1
    class C(object):
        b = 2
        c = A.B.a + b 
AttributeError: class A has no attribute 'B'

Does someone have an idea of how to use it? Thanks

Graham · Accepted Answer · 2017-09-27 04:19:20Z

1

The problem is that the class template (A) is not constructed while you're calling A.B.a. That is, A is not bound yet to a class.

Try this workaround:

class A():
    class B():
        a = 1

Now create C separately (A is already defined):

class C():
    b = 2
    c = A.B.a + b

And reference C from A:

A.C = C

This can possibly be done via meta-classes, but could be an over-kill here.

edited Sep 27, 2017 at 4:19

Graham

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answered Sep 1, 2016 at 11:55

Reut Sharabani

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1 Comment

Nikigv Over a year ago

I'm trying to avoid creating a separate class, I'm trying to stick the formation of nested classes.

Vishvajit Pathak · Accepted Answer · 2016-09-01 12:58:21Z

0

At compile time, the class definition for class A is not complete hence you can not access the classes, variables and methods defined in a parent class inside a nested class.

You can try separating the class definitions though as suggested by @Reut Sharabani.

answered Sep 1, 2016 at 12:58

Vishvajit Pathak

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Comments

napuzba · Accepted Answer · 2016-09-01 13:28:38Z

0

You can not access the class by its name, while the class definition statement is still executed.

class A(object):
   class B(object):
       a = 1
   class C(object):
       b = 2
       c = A.B.a + b # here class A statement is still executed, there is no A class yet

To solve the problem you must defer the execution of those statements :

move the all those statements to a classmethod
call them after the classes was defined.

class A(object):
    class B(object):
        @classmethod
        def init(cls):
            cls.a = 1
    class C(object):
        @classmethod
        def init(cls):
            cls.b = 2
            cls.c = A.B.a + cls.b 

    @classmethod
    def init(cls):
        cls.B.init()
        cls.C.init()

A.init()

edited Sep 1, 2016 at 13:28

answered Sep 1, 2016 at 13:23

napuzba

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