Why Doesn’t `FileHandler(“phonebook.txt”)` Automatically Add Data to the PhoneBook?

[Rajeev Bagra]

Context

While working through the Application Development section of the MOOC course, you may encounter the following constructor in a phone book application:

class PhoneBookApplication:
def __init__(self):
self.__phonebook = PhoneBook()
self.__filehandler = FileHandler("phonebook.txt")

# add the names and numbers from the file to the phone book
for name, numbers in self.__filehandler.load_file().items():
for number in numbers:
self.__phonebook.add_number(name, number)

A common and very reasonable question is:

Why isn’t this line enough?

self.__filehandler = FileHandler("phonebook.txt")

Shouldn’t it automatically add everything from the file into the phonebook?

Let’s clear up this confusion step by step.

---

Short Answer

Creating a FileHandler object does not automatically read the file or modify the phonebook.

It only:

• Stores the filename
• Prepares the object for future file operations

Nothing is added unless you explicitly tell the program to do so.

---

What Actually Happens When the Program Starts

Step 1: Creating an empty phonebook

self.__phonebook = PhoneBook()

• A new PhoneBook object is created
• It contains no names or numbers

---

Step 2: Creating the file handler

self.__filehandler = FileHandler("phonebook.txt")

This line:

• Creates a FileHandler object
• Stores "phonebook.txt" internally

It does not:

• Read the file
• Parse its contents
• Add anything to the phonebook

At this point:

• The phonebook is empty
• The file handler just knows which file to work with

---

What load_file() Really Does

The load_file() method is responsible for:

• Opening the file
• Reading its contents
• Converting the data into a dictionary
• Returning that dictionary

Example return value:

{
"Alice": ["12345", "67890"],
"Bob": ["55555"]
}

Importantly:

load_file() returns data — it does not store it anywhere else.

---

Why We Need Extra Code to Connect File and PhoneBook

This part is crucial:

for name, numbers in self.__filehandler.load_file().items():
for number in numbers:
self.__phonebook.add_number(name, number)

Here’s what this code does:

1. Takes raw data from the file
2. Loops through each name and number
3. Uses PhoneBook’s public method (add_number)
4. Stores the data correctly inside the phonebook

Without this loop:

• The file is read
• The data exists
• But nothing is connected

---

Why Isn’t This Logic Inside FileHandler?

This is intentional and follows good software design principles.

Separation of Responsibilities

Each class has one clear job:

Class	Responsibility
FileHandler	Read and write files
PhoneBook	Store and manage contacts
PhoneBookApplication	Coordinate between components

If FileHandler started adding data to PhoneBook:

• It would need to know how the phonebook works internally
• The code would become tightly coupled
• Reusability and maintainability would suffer

---

A Simple Real-World Analogy

• FileHandler is a delivery person 📦
• PhoneBook is a storage shelf 📚
• Application is you 👤

The delivery person brings the package.
You decide:

• where to open it
• how to organize the contents
• where each item should go

The delivery person does not organize your shelves.

---

Why Not Read the File Inside the Constructor Automatically?

Putting heavy logic inside a constructor is usually a bad idea because:

• Constructors should only initialize state
• Side effects make testing harder
• Errors during object creation become harder to handle

Good design rule:

Constructors set things up — they don’t do the work.

---

Key Takeaway

Creating an object does not mean it performs actions automatically.

Objects:

• Hold state
• Expose methods
• Do nothing until those methods are called

The extra code exists because connecting components is the job of the application, not the file handler.

---

One-Line Summary for Learners

FileHandler reads data, PhoneBook stores data, and PhoneBookApplication connects the two — each class does exactly one job.

---

[Author]

Posts