From Dictionary of Lists to Dictionary of Objects: Phone Book Explained (Python MOOC 2025)

[Rajeev Bagra]

🔹 Initial Context

In the Python Programming MOOC 2025, Part 10 covers building larger applications with object-oriented design — including the Phone Book project.

In earlier parts, we stored phone numbers like this:

all_persons = {}

# Example:
all_persons["Eric"] = ["02-123456", "045-4356713"]
all_persons["Emily"] = ["040-324344"]

This is a dictionary of lists.

The MOOC course now asks you to change this so that the dictionary stores objects instead of lists — a key step in object-oriented programming.

👉 View the official MOOC page here:
📄 Phone Book with Objects in a Dictionary
https://programming-25.mooc.fi/part-10/4-application-development/ (programming-25.mooc.fi)

And here’s the main Part 10 overview:
📄 Developing a larger application (Part 10)
https://programming-25.mooc.fi/part-10/ (programming-25.mooc.fi)

---

❓ Q1: What’s the limitation of using a dictionary of lists?

Answer:
A dictionary of lists only stores raw data. That becomes hard to manage when new features (like addresses) are needed. It mixes data with logic and does not support encapsulation.

---

❓ Q2: What does “objects in a dictionary” mean?

Answer:
Instead of:

name → list of numbers

You have:

name → Person object

And the Person object holds all the data and the methods to manage it:

👉 Official reference in MOOC:
https://programming-25.mooc.fi/part-10/4-application-development/ (programming-25.mooc.fi)

---

❓ Q3: What is the Person class?

Answer:
It encapsulates name, phone numbers, and address:

class Person:
    def __init__(self, name: str):
        self.__name = name
        self.__numbers = []
        self.__address = None

    def name(self):
        return self.__name

    def numbers(self):
        return self.__numbers

    def address(self):
        return self.__address

    def add_number(self, number: str):
        self.__numbers.append(number)

    def add_address(self, address: str):
        self.__address = address

---

❓ Q4: How does the phone book dictionary change?

Answer:
Instead of storing lists:

all_persons["Eric"] = ["02-123456"]

You store objects:

all_persons["Eric"] = Person("Eric")
all_persons["Eric"].add_number("02-123456")

---

❓ Q5: How does the PhoneBook class update?

Answer:
It now creates and manages Person objects:

class PhoneBook:
    def __init__(self):
        self.__persons = {}

    def add_number(self, name, number):
        if name not in self.__persons:
            self.__persons[name] = Person(name)
        self.__persons[name].add_number(number)

    def add_address(self, name, address):
        if name not in self.__persons:
            self.__persons[name] = Person(name)
        self.__persons[name].add_address(address)

    def get_entry(self, name):
        return self.__persons.get(name)

---

❓ Q6: Why does the UI hardly change?

Answer:
Because the UI calls methods (add_number, add_address, get_entry) that haven’t changed — only the internal data structure has. This follows the separation of concerns principle.

Official MOOC guidance on separation of concerns:
https://programming-25.mooc.fi/part-10/4-application-development/ (programming-25.mooc.fi)

---

❓ Q7: How does searching work now?

Answer:
Instead of returning a list, the UI retrieves a Person object:

person = phonebook.get_entry(name)

if person is None:
    print("number unknown")
else:
    for number in person.numbers():
        print(number)

---

❓ Q8: Why should you take baby steps in refactoring?

Answer:
Because structural changes affect many places. The course recommends testing each step after making a change to prevent hard-to-find bugs.

MOOC reference:
https://programming-25.mooc.fi/part-10/4-application-development/ (programming-25.mooc.fi)

---

❓ Q9: How is this similar to the ExerciseCounter example?

Answer:
Both store objects in a dictionary instead of raw data:

students[name] = ExerciseCounter()
students[name].done()

This pattern is exactly what the phone book exercise teaches — replace “list of simple data” with “object with behavior”.

---

📌 Final Takeaway

✔ Objects help manage growing complexity
✔ A dictionary can hold objects, not just lists
✔ Separation of concerns improves maintainability
✔ Refactor step-by-step to avoid bugs

---

🔗 Useful MOOC URLs

📄 Phone Book with Objects — MOOC 2025 (Part 10.4)
https://programming-25.mooc.fi/part-10/4-application-development/ (programming-25.mooc.fi)

📄 Part 10 Overview — Python MOOC 2025
https://programming-25.mooc.fi/part-10/ (programming-25.mooc.fi)

---

[Author]

Posts