Lesson 22: Python Encapsulation, Inner Classes, and File Handling

Phase 1 of 8

Lesson Introduction

Welcome to Lesson 22! This lesson covers three powerful topics that every serious Python programmer needs to understand deeply.

We start with Encapsulation · the art of protecting your data inside a class so it cannot be changed accidentally from outside. Then we move to Inner Classes · classes that live inside other classes, useful for grouping related things together neatly. Finally, we open the door to one of the most practical programming skills of all: File Handling · how Python reads files stored on your computer.

By the end of this lesson you will be able to:

Understand what encapsulation is, why it matters, and how to use private and protected properties in Python
Create getter and setter methods to safely read and change private data
Understand name mangling · how Python secretly renames private attributes
Create inner classes and understand how they relate to outer classes
Open a file using Python's built-in open() function
Read the full contents of a file, read specific numbers of characters, and read files line by line
Use the with statement to open files safely and automatically close them

This lesson connects directly to the real world: from banking systems that protect your account balance, to software that reads and processes log files, sensor data, or student records.

✏️ Your Task

Practise what you just learned about Lesson Introduction. Open your editor, type the examples above by hand, modify them, and observe what changes.

Phase 2 of 8

Prerequisite Concepts

Before starting, make sure you are comfortable with these ideas from earlier lessons:

Classes and Objects · you know how to define a class, create an object, and use self
The __init__ method · you know how to set up a class with starting properties
Class methods · you know how to define functions inside a class
Inheritance and Polymorphism · you have seen how classes can share and override behaviour (Lessons 20 · 21)

If any of these feel shaky, quickly re-read those lessons before continuing. This lesson builds directly on them.

✏️ Your Task

Practise what you just learned about Prerequisite Concepts. Open your editor, type the examples above by hand, modify them, and observe what changes.

Phase 3 of 8

Part 1: Python Encapsulation

1.1 What Is Encapsulation?

Let us start with a real-life story.

Imagine you walk into a bank in Lagos and ask the teller to see a customer's account balance. The teller refuses · that information is protected. You cannot just reach into the filing cabinet and read it yourself. Instead, you must go through the proper channels: fill a form, verify your identity, and only then does the bank give you what you are allowed to see.

This is exactly what encapsulation does in programming.

Encapsulation means keeping data (properties) and methods (functions) bundled inside a class, while controlling how that data can be accessed or changed from outside the class.

Why does it exist?

Without encapsulation, any part of your program can directly change any variable in your class · even by accident. Imagine a Student class where the student's grade can be changed to 500 by mistake. Encapsulation stops this by protecting the grade behind a controlled gate.

What problem does it solve?

It protects data from:

Accidental changes (a bug sets age to a negative number)
Invalid values (someone sets a student's score to 999 out of 100)
Unauthorised access (outside code reading sensitive information)

How does it work?

Python uses special naming rules to mark properties as private or protected.

1.2 Public, Protected, and Private Properties

Python has three levels of access for class properties:

Level	Prefix	Meaning
Public	`name`	Anyone can access it
Protected	`_name`	Intended for internal/subclass use only
Private	`__name`	Strongly restricted · only the class itself can use it

Let us explore each one.

1.3 Public Properties (No Prefix)

A normal property with no underscore is public. Anyone can read or change it freely.

python

class Student:
    def __init__(self, name):
        self.name = name  # Public property — anyone can access this

s = Student("Amaka")
print(s.name)       # Access directly from outside — this is fine
s.name = "Ngozi"    # Change directly — also fine for public data
print(s.name)

Expected output in terminal:

code

Amaka
Ngozi

Public properties are simple and fine for data that does not need protection.

1.4 Protected Properties (Single Underscore `_`)

A single underscore _ before a property name signals: "This is intended for internal use. Be careful with it."

python

class BankAccount:
    def __init__(self, owner, balance):
        self.owner = owner
        self._balance = balance  # Protected — a gentle warning, not enforced

account = BankAccount("Emeka", 50000)
print(account.owner)     # Fine — public
print(account._balance)  # Works, but it is a signal: "you shouldn't do this"

Expected output in terminal:

code

Emeka
50000

Important: A single underscore is only a convention · a polite message to other programmers. Python does NOT actually stop anyone from accessing _balance. It is like a sign on a door that says "Staff Only" · people can still walk in, but they know they probably shouldn't.

1.5 Private Properties (Double Underscore `__`)

A double underscore __ before a property name makes it private. Python enforces this much more strongly using a technique called name mangling (we will explain this shortly).

python

class Person:
    def __init__(self, name, age):
        self.name = name       # Public
        self.__age = age       # Private — cannot be accessed directly from outside

p1 = Person("Tunde", 30)
print(p1.name)    # Works fine
print(p1.__age)   # This will CAUSE AN ERROR

Expected output in terminal:

code

Tunde
AttributeError: 'Person' object has no attribute '__age'

The line print(p1.__age) crashes because Python has hidden __age under a different internal name. This is intentional · Python is protecting your data.

Think about it: Why did __age cause an error but _balance did not? The double underscore triggers Python's name mangling system, which we explore in Section 1.8.

1.6 Getter Methods · Reading Private Data Safely

Since private properties cannot be read directly from outside, we create a getter method · a specially named function inside the class that returns the private value.

What is a getter? It is simply a method that "gets" (retrieves) the value of a private property and hands it to whoever asks, in a controlled way.

python

class Person:
    def __init__(self, name, age):
        self.name = name
        self.__age = age       # Private property

    def get_age(self):         # Getter method
        return self.__age      # Returns the private value safely

p1 = Person("Chioma", 25)
print(p1.name)         # Direct access to public property — fine
print(p1.get_age())    # Using the getter to access private age — safe and controlled

Expected output in terminal:

code

Chioma
25

Line by line explanation:

self.__age = age · stores age as a private property; the double underscore hides it
def get_age(self) · defines a method inside the class; only this method can touch __age
return self.__age · the getter reads __age from inside the class (where it is allowed) and returns it
p1.get_age() · from outside the class, we call the getter and receive the age safely

1.7 Setter Methods · Changing Private Data Safely (With Validation)

A setter method "sets" (changes) the value of a private property. The setter's real power is that it can validate the new value before saving it, rejecting any bad input.

Why is this powerful? Without a setter, if someone types p1.__age = -5, age becomes negative · which makes no real-world sense. With a setter, you write rules that reject invalid input.

python

class Person:
    def __init__(self, name, age):
        self.name = name
        self.__age = age

    def get_age(self):
        return self.__age

    def set_age(self, age):         # Setter method
        if age > 0:                 # Validate: age must be a positive number
            self.__age = age        # Only update if the value is valid
        else:
            print("Age must be a positive number!")  # Reject invalid input

p1 = Person("Tobi", 25)
print(p1.get_age())     # 25

p1.set_age(26)          # Valid change
print(p1.get_age())     # 26

p1.set_age(-10)         # Invalid — should be rejected
print(p1.get_age())     # Still 26 — the bad value was blocked

Expected output in terminal:

code

25
26
Age must be a positive number!
26

This is encapsulation at work: the data is protected, changes go through a validation gate, and bad values are rejected cleanly.

1.8 Practical Encapsulation with Validation · A Student Grade System

Let us see encapsulation solving a real problem: protecting a student's grade so that no invalid score can be saved.

python

class Student:
    def __init__(self, name):
        self.name = name
        self.__grade = 0          # Private grade, starts at 0

    def set_grade(self, grade):
        if 0 <= grade <= 100:     # Grade must be between 0 and 100
            self.__grade = grade
        else:
            print("Grade must be between 0 and 100!")

    def get_grade(self):
        return self.__grade

    def get_status(self):
        if self.__grade >= 50:    # Nigerian passing mark
            return "Passed"
        else:
            return "Failed"

# Testing the Student class
student = Student("Babajide")
student.set_grade(78)
print(f"Name: {student.name}")
print(f"Grade: {student.get_grade()}")
print(f"Status: {student.get_status()}")

student.set_grade(120)           # Invalid — should be rejected
print(f"Grade after invalid attempt: {student.get_grade()}")  # Still 78

Expected output in terminal:

code

Name: Babajide
Grade: 78
Status: Passed
Grade must be between 0 and 100!
Grade after invalid attempt: 78

Real-world connection: School management systems, university portals (like JUPEB or JAMB), and hospital patient record systems all use encapsulation to prevent invalid data from entering their databases.

1.9 Private Methods · Hiding Internal Logic

Just like private properties, you can also make methods private. A private method can only be called from inside the class itself · not from outside.

Why would you want this? Sometimes a method is just a helper that does internal work that outside code should never call directly. Think of it as a factory machine's internal gears · the public only presses the button (public method), not the individual gears (private methods).

python

class Calculator:
    def __init__(self):
        self.result = 0

    def __validate(self, num):         # Private method — only used inside the class
        if not isinstance(num, (int, float)):
            return False               # Not a valid number
        return True

    def add(self, num):                # Public method — anyone can call this
        if self.__validate(num):       # Calls the private validator internally
            self.result += num
        else:
            print(f"Invalid input: {num} is not a number!")

# Using the Calculator
calc = Calculator()
calc.add(10)
calc.add(5.5)
calc.add("hello")         # Invalid — will be caught
print(calc.result)        # Should be 15.5

# Trying to call the private method directly
calc.__validate(5)        # This will CAUSE AN ERROR

Expected output in terminal:

code

Invalid input: hello is not a number!
15.5
AttributeError: 'Calculator' object has no attribute '__validate'

The __validate method is invisible from outside the class · exactly as intended.

1.10 Name Mangling · The Secret Behind `__`

You now know that __age causes an AttributeError when accessed from outside the class. But here is an interesting technical detail: Python does NOT delete __age. Instead, it renames it automatically.

When you write __age inside a class called Person, Python secretly renames it to _Person__age. This is called name mangling.

python

class Person:
    def __init__(self, name, age):
        self.name = name
        self.__age = age          # Python internally stores this as _Person__age

p1 = Person("Kemi", 30)

# Normal access — causes an error
# print(p1.__age)               # AttributeError

# Name mangling — accessing the renamed version (NOT recommended!)
print(p1._Person__age)           # This works, but it is bad practice

Expected output in terminal:

code

Important: Even though name mangling lets you access _Person__age, you should never do this in real code. It defeats the entire purpose of encapsulation. The rule exists only so Python's own internal systems (like inheritance) can still function properly.

1.11 Summary: Why Use Encapsulation?

Benefit	Explanation
Data Protection	Private data cannot be changed accidentally from outside
Validation	Setters can reject invalid values before they are stored
Flexibility	You can change internal logic later without breaking external code
Control	You decide exactly what outside code is allowed to see and change

✏️ Your Task

` Expected output in terminal: ` 30 ` > Important: Even though name mangling lets you access _Person__age, you should never do this in real code. It defeats the entire purpose of encapsulation. The rule exists only so Python's own internal systems (like inheritance) can still function properly. ·

Phase 4 of 8

Part 2: Python Inner Classes

2.1 What Is an Inner Class?

An inner class is a class that is defined inside another class.

Think of it like a house (outer class) that contains rooms (inner classes). The rooms exist inside the house and belong to it. Rooms are not visible from the street · you have to enter the house first.

Why do inner classes exist?

When a class is only useful inside one specific class and nowhere else, it makes sense to define it inside that class. This keeps your code organised and makes it clear that the inner class is a private helper belonging to the outer class.

2.2 Defining a Basic Inner Class

python

class Outer:
    def __init__(self):
        self.name = "I am the Outer Class"

    class Inner:                           # Inner class defined inside Outer
        def __init__(self):
            self.name = "I am the Inner Class"

        def display(self):
            print("Hello from the inner class!")

# Create an object of the outer class
outer = Outer()
print(outer.name)    # Access the outer class property directly

Expected output in terminal:

code

I am the Outer Class

Notice that printing outer.name only gives us the outer class's name. The inner class has not been used yet · it is just defined, waiting to be accessed.

2.3 Accessing the Inner Class from Outside

To use the inner class, you first create an outer object, then use that outer object to create an inner object.

python

class Outer:
    def __init__(self):
        self.name = "Outer"

    class Inner:
        def __init__(self):
            self.name = "Inner"

        def display(self):
            print("Hello from the inner class!")

# Step 1: Create an object of the outer class
outer = Outer()

# Step 2: Use the outer object to create an inner class object
inner = outer.Inner()    # Note the syntax: outer_object.InnerClassName()

# Step 3: Now use the inner object normally
inner.display()
print(inner.name)

Expected output in terminal:

code

Hello from the inner class!
Inner

Line by line:

outer = Outer() · creates the outer object as usual
inner = outer.Inner() · creates an inner class object; notice we access Inner through the outer object
inner.display() · calls the method on the inner object

2.4 Passing the Outer Object to the Inner Class

By default, an inner class cannot automatically see the outer class's properties. If you want the inner class to access the outer class's data, you must pass the outer object in as a parameter.

python

class Outer:
    def __init__(self):
        self.name = "Emeka"           # Outer property

    class Inner:
        def __init__(self, outer):    # Accept the outer object as parameter
            self.outer = outer        # Store the reference

        def display(self):
            print(f"The outer class name is: {self.outer.name}")  # Access outer data

# Create outer object
outer = Outer()

# Pass the outer object into the inner class
inner = outer.Inner(outer)    # outer is passed as the argument

inner.display()

Expected output in terminal:

code

The outer class name is: Emeka

Think about it: Why do you have to pass outer explicitly? Because unlike some other languages, Python inner classes do NOT automatically share a reference with their outer class. You must make the connection manually.

2.5 A Practical Example · Car and Engine

Inner classes are very useful when you have a "contains" relationship. For example, a car contains an engine. The engine on its own means nothing · it only makes sense inside a car.

python

class Car:
    def __init__(self, brand, model):
        self.brand = brand
        self.model = model
        self.engine = self.Engine()    # Create an Engine object as part of the Car

    class Engine:
        def __init__(self):
            self.status = "Off"        # Engine starts off

        def start(self):
            self.status = "Running"
            print("Engine started!")

        def stop(self):
            self.status = "Off"
            print("Engine stopped!")

    def drive(self):
        if self.engine.status == "Running":
            print(f"Driving the {self.brand} {self.model}. Vroom!")
        else:
            print("Start the engine first!")

# Creating and using the Car
toyota = Car("Toyota", "Camry")

toyota.drive()              # Engine is off — cannot drive yet
toyota.engine.start()       # Start the engine
toyota.drive()              # Now we can drive
toyota.engine.stop()        # Stop engine

Expected output in terminal:

code

Start the engine first!
Engine started!
Driving the Toyota Camry. Vroom!
Engine stopped!

Notice how the Engine class is defined inside Car. From outside, you access it as toyota.engine.start(). This feels natural · you are saying "the car's engine, start."

2.6 Multiple Inner Classes

A single outer class can contain multiple inner classes.

python

class Computer:
    def __init__(self):
        self.cpu = self.CPU()    # Create a CPU object
        self.ram = self.RAM()    # Create a RAM object

    class CPU:
        def process(self):
            print("CPU is processing data...")

    class RAM:
        def store(self):
            print("RAM is storing data temporarily...")

# Using the Computer
my_pc = Computer()
my_pc.cpu.process()
my_pc.ram.store()

Expected output in terminal:

code

CPU is processing data...
RAM is storing data temporarily...

This pattern is useful when modelling systems that have multiple distinct components · a computer with a CPU and RAM, a hospital with a Reception and a Laboratory, or an e-commerce order with a Cart and a Payment system.

2.7 A Nigerian Context Example · Hospital System

python

class Hospital:
    def __init__(self, name, location):
        self.name = name
        self.location = location
        self.reception = self.Reception()
        self.lab = self.Laboratory()

    class Reception:
        def check_in(self, patient_name):
            print(f"Welcome, {patient_name}! Please take a seat.")

    class Laboratory:
        def run_test(self, test_name):
            print(f"Running {test_name} test. Please wait...")

# Using the Hospital system
lasuth = Hospital("LASUTH", "Ikeja, Lagos")
lasuth.reception.check_in("Adaeze Okafor")
lasuth.lab.run_test("Malaria Parasite")

Expected output in terminal:

code

Welcome, Adaeze Okafor! Please take a seat.
Running Malaria Parasite test. Please wait...

✏️ Your Task

Practise what you just learned about : Python Inner Classes. Open your editor, type the examples above by hand, modify them, and observe what changes.

Phase 5 of 8

Part 3: Python File Handling

3.1 Why File Handling Matters

So far, all the data in your programs disappears when the program stops running. Variables, lists, and class objects · they all live in the computer's memory and vanish when you close the program.

File handling solves this problem. It lets Python programs:

Read data saved in .txt, .csv, or other files
Save results and reports to files that persist after the program ends
Process large datasets (like patient records, sales data, or school results) stored on disk

In the real world, file handling is used in every domain: banks process transaction logs, schools process result sheets, weather stations log temperature data, and mobile apps store user preferences.

3.2 The Four File Modes

When you open a file in Python, you must tell Python what you intend to do with it. This is called the mode.

Mode	Symbol	Meaning
Read	`"r"`	Open file for reading. Error if file does not exist. (Default)
Append	`"a"`	Open file to add new content at the end. Creates file if missing.
Write	`"w"`	Open file to write (overwrite). Creates file if missing.
Create	`"x"`	Create a new file. Error if file already exists.

There are also two format modes you can combine:

Format	Symbol	Meaning
Text	`"t"`	Read or write as plain text. (Default)
Binary	`"b"`	Read or write as raw bytes (for images, PDFs, etc.)

3.3 The `open()` Function · The Gateway to Files

The open() function is Python's built-in way to open any file.

Syntax:

python

open(filename, mode)

The simplest usage:

python

f = open("demofile.txt")          # Opens in read mode ("r") and text mode ("t") by default

This is exactly the same as:

python

f = open("demofile.txt", "rt")    # "r" = read, "t" = text — these are the defaults

Important: If the file does not exist when using "r" mode, Python raises a FileNotFoundError. Always make sure the file exists before reading it.

3.4 Reading a File · The `read()` Method

Once you have opened a file, use the read() method to get all of its contents as one big string.

First, create the file to work with. Imagine this file called demofile.txt exists in the same folder as your Python script, with these contents:

code

Hello! Welcome to demofile.txt
This file is for testing purposes.
Good Luck!

Now let us read it:

python

f = open("demofile.txt")    # Open the file
content = f.read()           # Read ALL contents into a string
print(content)               # Print the contents
f.close()                    # Always close the file when done!

Expected output in terminal:

code

Hello! Welcome to demofile.txt
This file is for testing purposes.
Good Luck!

Line by line:

f = open("demofile.txt") · opens the file and returns a file object stored in f
f.read() · reads the entire content of the file as one string
print(content) · displays the content
f.close() · closes the file; very important! Unclosed files can waste resources or cause data loss

3.5 Reading a File from a Different Location

If the file is not in the same folder as your Python script, you must provide the full file path.

python

# On Windows:
f = open("C:\\Users\\Babatunde\\Documents\\demofile.txt")
print(f.read())
f.close()

# On Mac/Linux:
f = open("/home/babatunde/Documents/demofile.txt")
print(f.read())
f.close()

Note: On Windows, use double backslash \\ or a raw string r"C:\path\to\file.txt" to avoid Python misreading the backslash as an escape character.

3.6 The `with` Statement · The Safe Way to Open Files

The safest and most Pythonic way to open files is using the with statement. When you use with, Python automatically closes the file for you when the block finishes · even if an error occurs.

python

with open("demofile.txt") as f:     # File opens here
    content = f.read()               # Read the content
    print(content)
# File is automatically closed here — no need to call f.close()

Expected output in terminal:

code

Hello! Welcome to demofile.txt
This file is for testing purposes.
Good Luck!

Best Practice: Always use the with statement when working with files. It is cleaner, safer, and prevents common mistakes like forgetting to close the file.

3.7 Reading a Specific Number of Characters

Instead of reading the whole file at once, you can tell read() exactly how many characters to return.

python

with open("demofile.txt") as f:
    print(f.read(5))    # Read only the first 5 characters

Expected output in terminal:

code

Hello

"Hello" is the first 5 characters of "Hello! Welcome to demofile.txt". The rest of the file is not touched.

Another example:

python

with open("demofile.txt") as f:
    print(f.read(12))   # Read first 12 characters

Expected output in terminal:

code

Hello! Welcom

Think about it: What would happen if you called f.read(5) twice in a row? Try to predict the answer before looking at the next section!

3.8 Reading One Line at a Time · The `readline()` Method

The readline() method reads one line from the file each time you call it.

Reading one line:

python

with open("demofile.txt") as f:
    print(f.readline())    # Reads the first line

Expected output in terminal:

code

Hello! Welcome to demofile.txt

Reading two lines:

python

with open("demofile.txt") as f:
    print(f.readline())    # First line
    print(f.readline())    # Second line

Expected output in terminal:

code

Hello! Welcome to demofile.txt
This file is for testing purposes.

Each call to readline() moves forward by one line. Think of it like a cursor that moves down the page one line at a time.

3.9 Reading All Lines · Looping Through the File

To read the entire file line by line (useful for processing large files), loop through the file object directly using a for loop.

python

with open("demofile.txt") as f:
    for line in f:
        print(line)

Expected output in terminal:

code

Hello! Welcome to demofile.txt

This file is for testing purposes.

Good Luck!

Why are there blank lines between each line? Each line in the file already ends with a newline character (\n). Then print() adds another newline by default. Use print(line, end="") to remove the extra blank lines.

python

with open("demofile.txt") as f:
    for line in f:
        print(line, end="")    # end="" prevents double spacing

Expected output in terminal:

code

Hello! Welcome to demofile.txt
This file is for testing purposes.
Good Luck!

3.10 Closing Files Manually

If you are not using the with statement, you must close the file manually using f.close().

python

f = open("demofile.txt")
print(f.readline())
f.close()               # ALWAYS close the file when you are done

Expected output in terminal:

code

Hello! Welcome to demofile.txt

Why is closing important? When a file is open, the operating system "locks" it in memory. Unclosed files waste memory, can cause data not to be saved properly, and may prevent other programs from accessing the same file. Always close files when you are done · or better, use with and let Python handle it.

✏️ Your Task

Practise what you just learned about : Python File Handling. Open your editor, type the examples above by hand, modify them, and observe what changes.

Phase 6 of 8

Guided Practice Exercises

🎯 Your Challenge

Exercise 1: Encapsulation · Building a Bank Account

Objective: Apply encapsulation to protect sensitive financial data.

Scenario: You are building a simple bank account system for First Bank Nigeria. The account balance must be private. No one should be able to directly change the balance · all deposits and withdrawals must go through validated methods.

Steps:

Create a BankAccount class with:

- A public property: owner (the account holder's name) - A private property: __balance (starting balance)

Add a getter get_balance() that returns the balance
Add a method deposit(amount) that adds to the balance (reject amounts ≤ 0)
Add a method withdraw(amount) that subtracts from the balance (reject if amount > balance or ≤ 0)
Test your class with deposits and withdrawals

Hint: Use if amount > 0 to validate before changing __balance.

Solution:

✏️ Task

Practise what you just learned about Guided Practice Exercises. Open your editor, type the examples above by hand, modify them, and observe what changes.

python

class BankAccount:
    def __init__(self, owner, initial_balance):
        self.owner = owner
        self.__balance = initial_balance    # Private — cannot be changed directly

    def get_balance(self):
        return self.__balance

    def deposit(self, amount):
        if amount > 0:
            self.__balance += amount
            print(f"Deposited ₦{amount:,}. New balance: ₦{self.__balance:,}")
        else:
            print("Deposit amount must be greater than zero!")

    def withdraw(self, amount):
        if amount <= 0:
            print("Withdrawal amount must be greater than zero!")
        elif amount > self.__balance:
            print("Insufficient funds!")
        else:
            self.__balance -= amount
            print(f"Withdrew ₦{amount:,}. New balance: ₦{self.__balance:,}")

# Testing the BankAccount
account = BankAccount("Ngozi Adeyemi", 100000)
print(f"Account owner: {account.owner}")
print(f"Opening balance: ₦{account.get_balance():,}")

account.deposit(50000)
account.withdraw(20000)
account.withdraw(200000)     # Should fail — insufficient funds
account.deposit(-5000)       # Should fail — invalid amount
print(f"Final balance: ₦{account.get_balance():,}")

Expected output in terminal:

code

Account owner: Ngozi Adeyemi
Opening balance: ₦100,000
Deposited ₦50,000. New balance: ₦150,000
Withdrew ₦20,000. New balance: ₦130,000
Insufficient funds!
Deposit amount must be greater than zero!
Final balance: ₦130,000

Self-check questions:

What happens if you try account.__balance? Why?
Could you change the balance to 999999999 without going through deposit()? Why not?
What makes deposit() and withdraw() better than direct access?

Exercise 2: Inner Classes · University Department System

Objective: Use inner classes to model a university with departments.

Scenario: University of Lagos (UNILAG) needs a class to represent a department. Each department has a Lecturer inner class and a Course inner class.

Steps:

Create a Department class with properties: name and faculty
Create an inner class Lecturer with: lecturer_name and a teach() method
Create an inner class Course with: course_title, course_code, and a describe() method
Pass the outer Department object to Lecturer so the lecturer knows which department they belong to
Create a Department, some Lecturers, and some Courses, and print information about them

Solution:

python

class Department:
    def __init__(self, name, faculty):
        self.name = name
        self.faculty = faculty

    class Lecturer:
        def __init__(self, lecturer_name, outer_dept):
            self.lecturer_name = lecturer_name
            self.department = outer_dept       # Reference to the outer class

        def teach(self):
            print(f"Dr. {self.lecturer_name} is teaching in the {self.department.name} department.")

    class Course:
        def __init__(self, course_title, course_code):
            self.course_title = course_title
            self.course_code = course_code

        def describe(self):
            print(f"Course: {self.course_title} ({self.course_code})")

# Creating the Department
cs_dept = Department("Computer Science", "Science and Technology")

# Creating Lecturers (passing the outer department object)
lecturer1 = cs_dept.Lecturer("Adewale Ogundimu", cs_dept)
lecturer2 = cs_dept.Lecturer("Fatima Bello", cs_dept)

# Creating Courses
course1 = cs_dept.Course("Introduction to Python", "CSC 101")
course2 = cs_dept.Course("Data Structures", "CSC 202")

# Using everything
lecturer1.teach()
lecturer2.teach()
course1.describe()
course2.describe()

Expected output in terminal:

code

Dr. Adewale Ogundimu is teaching in the Computer Science department.
Dr. Fatima Bello is teaching in the Computer Science department.
Course: Introduction to Python (CSC 101)
Course: Data Structures (CSC 202)

Exercise 3: File Reading · Student Results Processor

Objective: Practice reading a file line by line and processing each line.

Scenario: Your school in Abuja keeps student results in a text file called results.txt. You need to read it and display each student's result.

Step 1: First create the file. In your terminal or text editor, create results.txt with this content:

code

Adaobi Nwosu: 85
Kayode Fashola: 72
Zainab Musa: 91
Emeka Obi: 65
Halima Yusuf: 55

Step 2: Write Python to read and process this file.

Steps:

Open results.txt using a with statement
Loop through the file line by line
Print each line cleanly (remove extra spaces with .strip())
Identify students who passed (score ≥ 50) and who failed

Solution:

python

# Reading and processing student results

print("=== STUDENT RESULTS ===\n")

with open("results.txt") as f:
    for line in f:
        line = line.strip()          # Remove any leading/trailing whitespace or newlines
        if line:                     # Only process non-empty lines
            # Split at the colon to get name and score
            parts = line.split(": ")
            name = parts[0]
            score = int(parts[1])

            if score >= 50:
                status = "PASSED"
            else:
                status = "FAILED"

            print(f"{name}: {score}/100 — {status}")

Expected output in terminal:

code

=== STUDENT RESULTS ===

Adaobi Nwosu: 85/100 — PASSED
Kayode Fashola: 72/100 — PASSED
Zainab Musa: 91/100 — PASSED
Emeka Obi: 65/100 — PASSED
Halima Yusuf: 55/100 — PASSED

Self-check questions:

What does .strip() do? What would happen without it?
Why does the loop not need f.close()?
How would you change the passing mark to 60?

Phase 7 of 8

Common Beginner Mistakes

Mistake 1: Trying to access a private property directly

Wrong:

python

class Person:
    def __init__(self, age):
        self.__age = age

p = Person(25)
print(p.__age)    # AttributeError!

Correct:

python

class Person:
    def __init__(self, age):
        self.__age = age

    def get_age(self):
        return self.__age   # Use a getter

p = Person(25)
print(p.get_age())    # Works correctly

Mistake 2: Confusing `_` (single underscore) with `__` (double underscore)

Common confusion: Students think _salary is the same as __salary. They are not. Single underscore is a convention with no enforcement. Double underscore triggers name mangling.

python

class Staff:
    def __init__(self, salary):
        self._salary = salary     # Only a convention — still accessible
        self.__secret = "hidden"  # Enforced — triggers name mangling

s = Staff(50000)
print(s._salary)    # Works (just a warning to developers)
print(s.__secret)   # AttributeError

Mistake 3: Forgetting to pass the outer object to the inner class

Wrong:

python

class Outer:
    def __init__(self):
        self.city = "Lagos"

    class Inner:
        def show(self):
            print(self.city)   # ERROR — Inner doesn't know about 'city'

o = Outer()
i = o.Inner()
i.show()    # AttributeError

Correct:

python

class Outer:
    def __init__(self):
        self.city = "Lagos"

    class Inner:
        def __init__(self, outer):
            self.outer = outer       # Store the outer reference

        def show(self):
            print(self.outer.city)   # Access through the stored reference

o = Outer()
i = o.Inner(o)    # Pass the outer object
i.show()          # Lagos

Mistake 4: Forgetting to close a file (when not using `with`)

Wrong:

python

f = open("myfile.txt")
print(f.read())
# Forgot to close! Bad practice.

Correct Option A · Manual close:

python

f = open("myfile.txt")
print(f.read())
f.close()    # Always close

Correct Option B · Using with (recommended):

python

with open("myfile.txt") as f:
    print(f.read())
# Automatically closed — no need to call f.close()

Mistake 5: Reading a file that does not exist

Wrong:

python

f = open("missing_file.txt")    # FileNotFoundError if the file doesn't exist

Correct: Always check the file exists, or handle the error:

python

try:
    with open("missing_file.txt") as f:
        print(f.read())
except FileNotFoundError:
    print("Sorry, the file was not found. Please check the filename.")

Mistake 6: Double-spacing in output when reading lines

Wrong (double spacing):

python

with open("myfile.txt") as f:
    for line in f:
        print(line)    # Each line already has \n, and print adds another

Correct:

python

with open("myfile.txt") as f:
    for line in f:
        print(line, end="")    # Prevents the extra blank line
        # OR
        print(line.strip())    # Strips the \n before printing

✏️ Your Task

` Correct Option A · Manual close: `python f = open("myfile.txt") print(f.read()) f.close() # Always close ` Correct Option B · Using with (recommended): `python with open("myfile.txt") as f: print(f.read()) # Automatically closed · no need to call f.close() ` ·

Phase 8 of 8

Quick Reference Card

code

ENCAPSULATION
─────────────────────────────────────────
Public property:    self.name = value
Protected property: self._name = value     (convention only)
Private property:   self.__name = value    (enforced by Python)

Getter:  def get_name(self): return self.__name
Setter:  def set_name(self, val):
             if valid: self.__name = val
             else: print("Error!")

Name mangling: __age → _ClassName__age

─────────────────────────────────────────
INNER CLASSES
─────────────────────────────────────────
class Outer:
    class Inner:
        def method(self): ...

outer = Outer()
inner = outer.Inner()          # Access inner class via outer object
inner.method()

# Sharing outer data with inner:
class Inner:
    def __init__(self, outer):
        self.outer = outer
inner = outer.Inner(outer)     # Pass outer object explicitly

─────────────────────────────────────────
FILE MODES
─────────────────────────────────────────
"r"  → Read (default)
"a"  → Append
"w"  → Write (overwrite)
"x"  → Create new file

"t"  → Text mode (default)
"b"  → Binary mode

─────────────────────────────────────────
FILE READING
─────────────────────────────────────────
# Open and read all
with open("file.txt") as f:
    print(f.read())

# Read specific characters
with open("file.txt") as f:
    print(f.read(10))

# Read one line
with open("file.txt") as f:
    print(f.readline())

# Read all lines (loop)
with open("file.txt") as f:
    for line in f:
        print(line.strip())

# Manual open and close
f = open("file.txt")
print(f.read())
f.close()
─────────────────────────────────────────

✏️ Your Task

Practise what you just learned about Quick Reference Card. Open your editor, type the examples above by hand, modify them, and observe what changes.

🏗️ Build It — Mini Project

Project Overview

In this project, you will combine encapsulation, inner classes, and file reading to build a simple student management system.

Project Overview

You will create a School class that:

Has a private __school_name property (encapsulation)
Contains a Student inner class with private __grade (encapsulation within inner class)
Loads student data from a file and displays a formatted report

Stage 1 · Setup: Build the Core Classes

starter.py

class School:
    def __init__(self, school_name, location):
        self.__school_name = school_name    # Private
        self.location = location

    def get_school_name(self):
        return self.__school_name

    class Student:
        def __init__(self, name, student_id):
            self.name = name
            self.student_id = student_id
            self.__grade = 0                # Private

        def set_grade(self, grade):
            if 0 <= grade <= 100:
                self.__grade = grade
            else:
                print(f"Invalid grade for {self.name}!")

        def get_grade(self):
            return self.__grade

        def get_status(self):
            if self.__grade >= 50:
                return "PASSED"
            return "FAILED"

        def display(self):
            print(f"  ID: {self.student_id} | Name: {self.name} | "
                  f"Grade: {self.__grade}/100 | Status: {self.get_status()}")

Lesson 22 complete! 🎉

You covered:

✅ Lesson Introduction
✅ Prerequisite Concepts
✅ Part 1: Python Encapsulation
✅ Part 2: Python Inner Classes
✅ Part 3: Python File Handling
✅ Guided Practice Exercises
✅ Common Beginner Mistakes
✅ Quick Reference Card

← Lesson 21 Lesson 23 →

Python Encapsulation, Inner Classes, and File Handling

👋 Welcome to Lesson 22

1.1 What Is Encapsulation?

1.2 Public, Protected, and Private Properties

1.3 Public Properties (No Prefix)

1.4 Protected Properties (Single Underscore _)

1.5 Private Properties (Double Underscore __)

1.6 Getter Methods · Reading Private Data Safely

1.7 Setter Methods · Changing Private Data Safely (With Validation)

1.8 Practical Encapsulation with Validation · A Student Grade System

1.9 Private Methods · Hiding Internal Logic

1.10 Name Mangling · The Secret Behind __

1.11 Summary: Why Use Encapsulation?

2.1 What Is an Inner Class?

2.2 Defining a Basic Inner Class

2.3 Accessing the Inner Class from Outside

2.4 Passing the Outer Object to the Inner Class

2.5 A Practical Example · Car and Engine

2.6 Multiple Inner Classes

2.7 A Nigerian Context Example · Hospital System

3.1 Why File Handling Matters

3.2 The Four File Modes

3.3 The open() Function · The Gateway to Files

3.4 Reading a File · The read() Method

3.5 Reading a File from a Different Location

3.6 The with Statement · The Safe Way to Open Files

3.7 Reading a Specific Number of Characters

3.8 Reading One Line at a Time · The readline() Method

3.9 Reading All Lines · Looping Through the File

3.10 Closing Files Manually

Exercise 1: Encapsulation · Building a Bank Account

Exercise 2: Inner Classes · University Department System

Exercise 3: File Reading · Student Results Processor

Mistake 1: Trying to access a private property directly

Mistake 2: Confusing _ (single underscore) with __ (double underscore)

Mistake 3: Forgetting to pass the outer object to the inner class

Mistake 4: Forgetting to close a file (when not using with)

Mistake 5: Reading a file that does not exist

Mistake 6: Double-spacing in output when reading lines

Project Overview

Stage 1 · Setup: Build the Core Classes

Lesson 22 complete! 🎉

1.4 Protected Properties (Single Underscore `_`)

1.5 Private Properties (Double Underscore `__`)

1.10 Name Mangling · The Secret Behind `__`

3.3 The `open()` Function · The Gateway to Files

3.4 Reading a File · The `read()` Method

3.6 The `with` Statement · The Safe Way to Open Files

3.8 Reading One Line at a Time · The `readline()` Method

Mistake 2: Confusing `_` (single underscore) with `__` (double underscore)

Mistake 4: Forgetting to close a file (when not using `with`)