Hello once again! Today's lesson is centered around leveraging the principles of Object-Oriented Programming (OOP) — Encapsulation, Abstraction, Polymorphism, and Composition — to enhance code readability and structure. Buckle up for an exciting journey ahead!

OOP principles act as a scaffold for building readable, maintainable, and flexible code — these are the characteristics we seek while refactoring. By creating logical groupings of properties and behaviors in classes, we foster a codebase that's easier to comprehend and modify. Let's put this into perspective as we progress.

Encapsulation involves bundling related properties and methods within a class, thereby creating an organization that mirrors the real world.

Suppose we possess scattered student information within our program.

student_name = "Alice"
student_age = 20
student_grade = 3.9

def display_student_info():
  print(f"Student Name: {student_name}")
  print(f"Student Age: {student_age}")
  print(f"Student Grade: {student_grade}")

def update_student_grade(new_grade):
  global student_grade
  student_grade = new_grade

Although functional, the code could cause potential confusion as the related attributes and behaviors aren't logically grouped. Let's encapsulate!

class Student:
  def __init__(self, name, age, grade):
    self.name = name
    self.age = age
    self.grade = grade

  def display_student_info(self):
    print(f"Student Name: {self.name}")
    print(f"Student Age: {self.age}")
    print(f"Student Grade: {self.grade}")

  def update_student_grade(self, new_grade):
    self.grade = new_grade

After refactoring, all student-related properties and methods are contained within the Student class, thereby enhancing readability and maintainability.

Next up is Abstraction. It is about exposing the relevant features and concealing the complexities.

Consider a code snippet calculating a student's grade point average (GPA) through complex operations:

def calculate_gpa(grades):
  total_points = 0
  grade_points = {'A': 4, 'B': 3, 'C': 2, 'D': 1, 'F': 0}
  for grade in grades:
    total_points += grade_points[grade]
  gpa = total_points / len(grades)
  return gpa

We can encapsulate this within the calculate_gpa() method of our Student class, thereby simplifying the interaction.

class Student:
  def __init__(self, name, grades):
    self.name = name
    self.grades = grades
    self.gpa = self.calculate_gpa()

  def calculate_gpa(self):
    total_points = 0
    grade_points = {'A': 4, 'B': 3, 'C': 2, 'D': 1, 'F': 0}
    for grade in self.grades:
      total_points += grade_points[grade]
    return total_points / len(self.grades)

We can now access the gpa as an attribute of the student object, which is calculated behind the scenes.