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in python, When assigning a variable to self variable in a class its a pointer to the self variable?

January 30, 2022

In the example code below, i have a piece of code from a project. I have a self.test_json.test_id, which is an instance of a class being assigned by a json in the constructor, and in the function do_somthing(self) i want to assign a new variable var in the function scope to be equal to the self.test_json.test_id variable, without changing it.
but when i call to do_somthing(self), when i change the value of var the value of self.test_json.test_id changes aswell. why this is happening? as far as i know there is no pointers and references in python, shouldn’t the var create a copy of self.test_json.test_id ?

from types import SimpleNamespace
import json


class Test:
    def __init__(self, json_data):
        self.tests = json_data.tests
        self.test_json = ""
        
    def do_something(self):
        for test in self.tests:
            var = test.test_json
            self.test_json = var
            print("The self.test_json is: " + str(self.test_json.test_id))
            print("test_json is: " + str(var.test_id))
            var.test_id = 6
            print("(should be 5, unchanged) The self.test_json is: " + str(self.test_json.test_id))
            print("(should be 6, changed) test_json is: " + str(var.test_id))


if __name__ == '__main__':
    json_data = b'{"tests": [{"test_json": {"test_id":5 }}]}'
    data_as_simple_namespace = json.loads(json_data, object_hook=lambda d: SimpleNamespace(**d))
    t = Test(data_as_simple_namespace)
    t.do_something()

after the function is called, it will print:

The self.test_json is: 5
test_json is: 5
(should be 5, unchanged) The self.test_json is: 6
(should be 6, changed) test_json is: 6

>Solution :

The self.test_json = var makes the self.test_json a reference to var, they are connected/referring to the same value/memory.

A quick fix could look this:

import copy  # HERE
import json

from types import SimpleNamespace


class Test:
    def __init__(self, json_data):
        self.tests = json_data.tests
        self.test_json = ""

    def do_something(self):
        for test in self.tests:
            var = test.test_json
            self.test_json = copy.copy(var)  # and HERE
            print("The self.test_json is: " + str(self.test_json.test_id))
            print("test_json is: " + str(var.test_id))
            var.test_id = 6
            print(
                "(should be 5, unchanged) The self.test_json is: "
                + str(self.test_json.test_id)
            )
            print("(should be 6, changed) test_json is: " + str(var.test_id))


if __name__ == "__main__":
    json_data = b'{"tests": [{"test_json": {"test_id":5 }}]}'
    data_as_simple_namespace = json.loads(
        json_data, object_hook=lambda d: SimpleNamespace(**d)
    )
    t = Test(data_as_simple_namespace)
    t.do_something()