Cant access instance variable from another class

Question

I have checked many questions replied here, but can't access an instance variable from another class(I have tried this as example)

#in file: view.py
class treeview():
    def __init__(self):
        ....(no mention of row_num)
    def viewer(self, booklist, act=-1):
        row_num = len(treeview().full_list)
        print(row_num)   # prints the correct value, which is > 0
        return row_num

#in main file 
class Window(Gtk.ApplicationWindow):
    def __init__(self, application, giofile=None):
        self.TreeView = view.treeview()
    def extract_data_from_cr(self, select_button):
        print(self.TreeView.row_num)

Gives error:

   AttributeError: 'treeview' object has no attribute 'row_num'

If I try to add row_num inside treeview() as:

class treeview():
    def __init__(self):
        self.row_num = 0
    def viewer(self, booklist, act=-1):
        self.row_num = len(treeview().full_list)
        print(self.row_num)
        # return row_num

then, print(self.TreeView.row_num) in main always yields 0.

I cant find whats going wrong here.Kindly help.

After Mitch's comment if I define row_num without calling treeview() again as:

class treeview():
    def __init__(self):
        self.row_num = 0
    def viewer(self, booklist, act=-1):
        self.row_num = len(self.bookstore)
        print(type(self.bookstore))  # <class 'gi.overrides.Gtk.ListStore'>

        print(self.row_num) # 5 (expected for a particular case)

Now, when calling the def extract_data_from_cr, I am expecting this number:

def extract_data_from_cr(self, select_button):
    print(self.TreeView.row_num)   #is giving 0

A MCVE

$cat mcve.py 
class classA():
    def __init__(self):
        self.row_num = 0
        print("HiA")
    def define(self):
        la = ["A", "B", "C"]
        self.row_num = len(la)
        print(self.row_num)  #prints 3 when get called from mcve2
        return(self.row_num)
classA().define()

and

$cat mcve2.py 
#!/usr/bin/python
import mcve

class classB():
    def get_val(self):
        self.MCVE = mcve.classA()
        print("Hi2")
        print(self.MCVE.row_num) #prints 0

classB().get_val()

Result:

 python3 mcve2.py 
HiA
3
HiA
Hi2
0

I know. if I call ClassA.define() explicitely from ClassB.get_val(), I will get the desired value. But I am looking for transfaring the value it got (Result: line 2)

Answer 1

^{Working from your edited MCVE:}

When you set self.MCVE = classA() in the get_val method, you are setting MCVE to be a new instance of classA. Hence, any modifications to the row_num attribute of some other instance of classA are irrelevant.

^e.g. classA().define() is modifying the row_num attribute for an entirely different class instance. Its row_num is an instance variable which is only defined for that particular instance.

Should you want the row_num attribute to persist across all classA instances, you would want to set it as a class variable, like so (bit of a nonsensical example though).

class classA():
    row_num = 0

    def __init__(self):
        print("HiA")
    def define(self):
        la = ["A", "B", "C"]
        classA.row_num = len(la)
        print(classA.row_num)  #prints 3 when get called from mcve2
        return classA.row_num
classA().define()

class classB():
    def get_val(self):
        self.MCVE = classA()
        print("Hi2")
        print(self.MCVE.row_num) #prints 0

classB().get_val()

Outputs:

HiA
3
HiA
Hi2
3

You otherwise may be looking to use a property, which could be set in the __init__ of the class.

Answer 2

You never declared row_num in the class. You probably meant to do

class treeview():
    def __init__(self):
        ....(no mention of row_num)
    def viewer(self, booklist, act=-1):
        self.row_num = len(treeview().full_list)
        print(self.row_num)   # prints the correct value
        return self.row_num

Which then works again fine.