Why We Use get_numbers() Instead of Accessing the Dictionary Directly (Python OOP)

[Rajeev Bagra]

📌 Context (Initial Code)

We have a PhoneBook class that stores names and phone numbers, and a PhoneBookApplication class that provides a text-based user interface (UI).

✅ PhoneBook (Core Logic)

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

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

    def get_numbers(self, name: str):
        if not name in self.__persons:
            return None
        return self.__persons[name]

✅ PhoneBookApplication (User Interface)

class PhoneBookApplication:
    def __init__(self):
        self.__phonebook = PhoneBook()

    def search(self):
        name = input("name: ")
        numbers = self.__phonebook.get_numbers(name)
        if numbers == None:
            print("number unknown")
            return
        for number in numbers:
            print(number)

---

✅ Q&A: Why Not Access __persons[name] Directly in search()?

❓ Q: Why don’t we write this inside search()?

numbers = self.__phonebook.persons[name]

✅ Answer:
Because persons is not a public attribute, and the dictionary is actually stored as:

self.__persons

inside the PhoneBook class, which is private due to the double underscore __.

So the UI class (PhoneBookApplication) cannot (and should not) directly access it.

---

❓ Q: What happens if we try to access the dictionary directly anyway?

✅ Answer:
If you try something like this:

numbers = self.__phonebook.__persons[name]

It will fail because __persons is private, and Python will raise an error like:

✅ AttributeError (cannot access private attribute)

Also, direct access like this would crash if the name doesn’t exist, causing a:

✅ KeyError

---

❓ Q: Why is using get_numbers(name) better?

✅ Answer:
Because get_numbers() is a public method designed to:

1. safely check if the name exists
2. return the numbers if found
3. return None if not found

That’s why the UI can handle missing names properly:

if numbers == None:
    print("number unknown")

No crash ✅

---

❓ Q: What programming principle does this follow?

✅ Answer:
This is called Encapsulation (and also supports Separation of Concerns).

It means:

• PhoneBook is responsible for storing and managing data
• PhoneBookApplication is responsible only for interacting with the user

So the UI should use methods like:

✅ add_number()
✅ get_numbers()

instead of touching internal variables directly.

---

❓ Q: What is the biggest advantage of doing it this way?

✅ Answer:
Your program becomes easy to maintain and improve.

For example, if later you change how data is stored (dictionary → database), you only update PhoneBook.

The UI code still works, because it only relies on:

self.__phonebook.get_numbers(name)

---

✅ Final Takeaway

We don’t access persons[name] directly in the UI because the phonebook data is private and should be managed only through public methods like get_numbers(), which also prevents errors and keeps the design clean.

[Author]

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