Python Crash Course (2nd edition) Exercise 9-8

Question

I have a question regarding the solution to Exercise 9-8 in Python Crash Course (2nd edition). The codes are as follows:

class User():
    """Represent a simple user profile."""

    def __init__(self, first_name, last_name, username, email, location):
        """Initialize the user."""
        self.first_name = first_name.title()
        self.last_name = last_name.title()
        self.username = username
        self.email = email
        self.location = location.title()
        self.login_attempts = 0

    def describe_user(self):
        """Display a summary of the user's information."""
        print(f"\n{self.first_name} {self.last_name}")
        print(f"  Username: {self.username}")
        print(f"  Email: {self.email}")
        print(f"  Location: {self.location}")

    def greet_user(self):
        """Display a personalized greeting to the user."""
        print(f"\nWelcome back, {self.username}!")

class Admin(User):
    """A user with administrative privileges."""

    def __init__(self, first_name, last_name, username, email, location):
        """Initialize the admin."""
        super().__init__(first_name, last_name, username, email, location)
        
        # Initialize an empty set of privileges.
        self.privileges = Privileges()


class Privileges():
    """A class to store an admin's privileges."""

    def __init__(self, privileges=[]):
        self.privileges = privileges

    def show_privileges(self):
        print("\nPrivileges:")
        if self.privileges:
            for privilege in self.privileges:
                print(f"- {privilege}")
        else:
            print("- This user has no privileges.")


eric = Admin('eric', 'matthes', 'e_matthes', 'e_matthes@example.com', 'alaska')
eric.describe_user()

eric.privileges.show_privileges()

print("\nAdding privileges...")
eric_privileges = [
    'can reset passwords',
    'can moderate discussions',
    'can suspend accounts',
    ]
eric.privileges.privileges = eric_privileges
eric.privileges.show_privileges()

My question is about the second last line. I understand the first "priviledges" refers to the instance in the class Admin. So, what about the second "priviledges"?

Many thanks in advance!

Answer 1

To answer your question, it's because Admin contains self.privileges = Privileges(), then the Privileges class contains self.privileges = privileges again.

I don't know why they've set it up like this though. The code is a tad sketchy. The Privileges class alone has multiple odd things:

• You don't need the () in class Privileges():.
• privileges=[] is nearly always wrong. Never have a mutable default argument.
• Why even have the class? All it's doing is housing a method. That method could easily be a method of Admin, or as a standalone function that accepts admin.privileges as an argument. It isn't encapsulating anything, so it's really just making the code more complex than it needs to be.

I'd be cautious taking advice from this source.

Answer 2

The class Admin has an attribute self.privileges that is an object of the class Privileges. The class Privileges also has an attribute named self.privileges.

class Admin, attribute privileges -> class Privileges, attribute privileges
eric.privileges.privileges = eric_privileges

Answer 3

class Privileges():
    """A class to store an admin's privileges."""

    def __init__(self, privileges=[]):
        self.privileges = privileges

not sure why but was thinking about this line of code,

adding this at the end of the file

print('***************')
for property, value in vars(eric).items():
    print(property, ":", value)
print('***************')

print(type(eric.privileges))
print(eric.privileges)
print('id : ',id(eric.privileges))
for property, value in vars(eric.privileges).items():
    print(property, ":", value)
print('***************')
print(type(eric.privileges.privileges))
print(eric.privileges.privileges)
print('id : ',id(eric.privileges.privileges))

gives :

***************
first_name : Eric
last_name : Matthes
username : e_matthes
email : e_matthes@example.com
location : Alaska
login_attempts : 0
privileges : <__main__.Privileges object at 0x7fd795456be0>
***************
<class '__main__.Privileges'>
<__main__.Privileges object at 0x7fd795456be0>
id :  140563899050976
privileges : ['can reset passwords', 'can moderate discussions', 'can suspend accounts']
***************
<class 'list'>
['can reset passwords', 'can moderate discussions', 'can suspend accounts']
id :  140563932329856

guess these two privileges are related but slightly different

As per answer one : as in one: The class Admin has an attribute self.privileges that is an object of the class Privileges. The class Privileges also has an attribute named self.privileges. class Admin, attribute privileges -> class Privileges, attribute privileges eric.privileges.privileges = eric_privileges