Why Does One Class Require an Argument While the Other Does Not?

[Rajeev Bagra]

❓ Question:

In the code below from my Python project:

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

I notice that PhoneBook() is created without arguments, while FileHandler("phonebook.txt") requires a filename argument.

Why is there a difference? Why doesn’t PhoneBook need an argument in its constructor?

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✅ Answer:

The difference is caused by how the two classes are designed, not by how they are used.

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1. PhoneBook() requires no arguments

The constructor of PhoneBook is defined as:

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

It takes only self, which Python supplies automatically.
No external data is required to create a phone book—it simply starts empty.

✔ The class can initialize itself internally
✔ No filename, settings, or input is needed
✔ Therefore, you call it as PhoneBook() with no arguments

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2. FileHandler("phonebook.txt") must receive an argument

The constructor is defined as:

class FileHandler:
def __init__(self, filename):
self.__filename = filename

This class cannot operate without knowing which file to read.
The filename argument is required so the object knows:

• which file to open
• where to load initial contact data from

If you try to create it without the filename:

FileHandler()

Python will raise an error:

TypeError: __init__() missing 1 required positional argument: 'filename'

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3. Summary

Class	Constructor	Requires Argument?	Reason
PhoneBook()	__init__(self)	❌ No	It starts with an empty dictionary, so no external info needed
FileHandler("phonebook.txt")	__init__(self, filename)	✔ Yes	It must know which file to read

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🟦 Final Explanation

You only pass arguments to a class when its constructor is designed to accept them.
FileHandler requires configuration (a filename); PhoneBook does not.

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