[FIXED] How can I access the changed variable from another class

Issue

I want to print(read or use) the changing variable self.tt in B().check() while class A is changing self.tt, is it any way to do it or other better solution to achieve the task?

class A:
    def __init__(self):
        self.tt = 0

    def change_tt(self):
        for i in range(100000000):
            self.tt += 1
            sleep(1)


class B():
    def __init__(self):
        self.x = A.tt

    def check(self):
        while True:
            print(self.x)


a = A()
b = B()

x1 = threading.Thread(target=A.change_tt)
x2 = threading.Thread(target=B.check)

x1.start()
x2.start()

AttributeError: type object 'A' has no attribute 'tt'

Solution

There are a couple of things going on here. The biggest issue is that you can't access an instance attribute from the class. A secondary issue is that integers are immutable.

Here is some code that should fix both:

class A:
    def __init__(self):
        self.tt = 0

    def change_tt(self):
        for i in range(100000000):
            self.tt += 1
            sleep(1)


class B():
    def __init__(self, a):
        self.a = a

    def check(self):
        while True:
            print(self.a.tt)


a = A()
b = B(a)

x1 = threading.Thread(target=a.change_tt)
x2 = threading.Thread(target=b.check)

x1.start()
x2.start()

Notice that the code of class A remains unchanged. However, it's important to understand that it is being used differently. A is the class object. It has attributes that are functions, but no integers. When you create instance a, the functions become methods when you access them with the . operator. a also has an attribute tt.

A thread target should be a no-arg callable. The functions A.change_tt and B.check both require a single positional argument, self. However, the bound methods a.change_tt and b.check are no-arg callables. The process of binding a function to an instance with the . operator creates a wrapper that passes in self automatically.

When you do self.tt += 1, the object that is the previous value of tt is unbound from tt and possibly garbage collected. Integers are immutable, which means that what really happens here is self.tt = self.tt + 1. That means that the statement self.x = A.tt in B.__init__ is unreasonable even if tt existed in A. x would be a reference to the object that is the initial value of tt, and would keep referring to that object even as tt changed to the incremented version.

An instance of B needs to know about the object that A.tt refers to currently. One way to do that is to pass a reference to B.__init__. That's why we define __init__(self, a), and invoke B as B(a) to get an instance that refers to a.

Answered By - Mad Physicist

Answer Checked By - Senaida (PHPFixing Volunteer)