Dictionary Logic in Python class

January 8, 2022

I was clear how dictionaries work in Python until I found this logic.

class Sample:
    __test = {}

    def __init__(self):
        self.__dict__ = self.__test
        self.key = "answer"
        self.count = "numbers"

    def print_test(self):
        print(self.__test)


s = Sample()

s.print_test()

In the above code a dictionary is initialized and then same variable is assigned to the class dict. In the next line we are initializing 2 more variables to the class.

In the end we are initializing Sample class to an "s object"

As per my understanding this should be following output and __test dictionary should be empty

s.count = "numbers"
s.key = "answers"

But to my surprise the print_test functions returns

{'count': 'numbers', 'key': 'answer'}

Can anyone explain how the __test dictionary got these items as key value pairs.

>Solution :

The __init__ function runs sequentially. When you set self.__dict__ = self.__test, you assign the object’s internal state dictionary to be empty. However, when you next set the values of self.key and self.count, those attributes are added to the now empty self.__dict__, and by mutual assignment, to self.__test

You can see the difference you assign the empty dict at the end of the __init__ method.

class Sample:
    __test = {}

    def __init__(self):
        self.key = "answer"
        self.count = "numbers"
        self.__dict__ = self.__test

    def print_test(self):
        print(self.__test)


s = Sample()

s.print_test()
# prints:
{}