How to store objects of custom class as session variables in django?

In my case i wants to store object of my USER class to be stored as session variable To be more precise my class contains several methods for USER structure of my USER class

class USER:
    def __init__(self , phone_number_of_user = None , user_name  = None):
        self.phone_number = phone_number_of_user
        self.user_name    = user_name
    def login(self):
        login_code_will_be_here
    def get_user_name(self):
        return self.user_name
    def set_data_of_user_in_firebase(self , data = None):
        code to set data of user will be here

firstly i create a USER object

user = USER(phone_number = 9******** , user_name = John)

But when i tries to store my objects in session of django like this

request.session['user'] = user

django shows me following error

Object of type USER is not JSON serializable