Global variable through classes?

Question

I'm trying to call a method of a class using a global variable, but there seems to be something wrong with my logic.

In the code below, A calls B which calls C which calls a method of B .

x = None

class A():

    def __init__(self):

        global x
        x = B()

class B():

    def __init__(self):

        C()

    def bla(self):

        print('bla')

class C():

    def __init__(self):

        global x
        x.bla()

A()

The error I'm getting:

AttributeError: 'NoneType' object has no attribute 'bla'

What am I missing?

Answer 1

When you do x = B() , the result of calling B() cannot be assigned to x until after B is finished initialized. But B.__init__() is called when you create the B instance, and it immediately calls C() . In other words, when you do x = B() , things happen in this order:

1. call B.__init__()
2. call C.__init__() (because of C() call in B.__init__() )
3. assign result of B() to x .

But step 3 never happens, because C.__init__() raises an error, because step 3 hasn't happened yet so the object hasn't been assigned to x yet.

It's not clear what you're trying to accomplish here, so it's hard to say how best to change your code. There is no way for C.__init__ to make use of a variable that will not be defined until after C.__init__ finishes running.

Answer 2

Here is your problem

you say x = b() well, x is None until b finishes initializing... which also means until c finishes initializing, which would cause a problem in class C when you do

x.bla()

Answer 3

This is because x is not completely initialized until it finishes calling the init function for each class. Here you tried to access global x, before completing its initialization.