如何在 python 中的类之间共享非全局变量

Question

I need to share a variable between 2 classes that are called in the main file. in the following replicable code, variable sharedVar is defined in main file and passed to both class child1 and child 2. I am expecting the variable to be equal to 2 in child2 since it has been changed from 1 to 2 in child1. but it still prints it as 1.

file mainClass:

from class1 import Class1
from class2 import Class2

class mainClass():
    sharedVar = 1
    my_child1 = Class1(sharedVar)  # expecting to print 1
    my_child2 = Class2(sharedVar)  # expecting to print 2

file class1

class Class1():
    def __init__(self,sharedVar):
        sharedVar += 1
        print("Class1",sharedVar) # output is 2

file class2

class Class2():
    def __init__(self,sharedVar):
        print("Class2",sharedVar) # output is 1 but expected to be 2

Note: These classes are in 3 separate files, so defining a global variable outside the classes will not work!

If you have an answer, please be specific and provide the code. writing an explanation without codes is not going to help me nor the future readers of this question.

Answer 1

Like this:

file mainClass:

from class1 import Class1
from class2 import Class2

class mainClass():
    def __init__(self):
        self.sharedVar = 1
        self.my_child1 = Class1(self)  # expecting to print 1
        self.my_child2 = Class2(self)  # expecting to print 2

file class1

class Class1():
    def __init__(self,sharedObj):
        sharedObj.sharedVar += 1
        print("Class1",sharedObj.sharedVar)

file class2

class Class2():
    def __init__(self,sharedObj):
        print("Class2",sharedObj.sharedVar)

Answer 2

if you came from a c++ or java background, of course, this is what you expect to happen(If they are passed by reference), however, it's not like that in python. variables here are a bit different, it's called identifier; when I say that x=5 this means python will create an object in memory of type int(let's call b1) and with value 5, and will let x point to that variable, so when I make x +=1 this mean that python will create a new object in memory (call it b2)of type int with value 6 and will let x now point to b2. so now b1 if left unpointed.

Another example: if we say x=5, y=x--> then we have an object in memory of type int with value 5, and x and y point to it; so when I say x=9 I let x point to another object which is equal to 9, but still y points to 5

so when you send it to class1 it let x point just to a different object and in class2 it send the original object which is equal to 1. I understood very deeply from Data structure and algorithms in python book I encourage you to read that section.