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Object-Oriented Programming in Python

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As mentioned in the first video, an object-oriented approach is most useful when your code involves complex interactions of many objects. In real production code, classes can have dozens of attributes and methods with complicated logic, but the underlying structure is the same as with the most simple class.

class Employee:
  
  def set_name(self, new_name):
    self.name = new_name
  
  # Add set_salary() method
  def set_salary(self, new_salary):
    self.salary = new_salary
  
# Create an object emp of class Employee  
emp = Employee()

# Use set_name to set the name of emp to 'Korel Rossi'
emp.set_name('Korel Rossi')

# Set the salary of emp to 50000
emp.set_salary(50000)

Try running dir(emp) in the console and see if you can find where these attributes and methods pop up!

class Employee:
    def set_name(self, new_name):
        self.name = new_name

    def set_salary(self, new_salary):
        self.salary = new_salary 

    def give_raise(self, amount):
        self.salary = self.salary + amount

    # Add monthly_salary method that returns 1/12th of salary attribute
    def monthly_salary(self):
        return self.salary / 12
    
emp = Employee()
emp.set_name('Korel Rossi')
emp.set_salary(50000)

# Get monthly salary of emp and assign to mon_sal
mon_sal = emp.monthly_salary()

# Print mon_sal
print(mon_sal)
# Import datetime from datetime
from datetime import datetime

class Employee:
    
    def __init__(self, name, salary=0):
        self.name = name
        if salary > 0:
          self.salary = salary
        else:
          self.salary = 0
          print("Invalid salary!")
          
        # Add the hire_date attribute and set it to today's date
        self.hire_date = datetime.today()
        
   # ...Other methods omitted for brevity ...
      
emp = Employee("Korel Rossi")
print(emp.name)
print(emp.hire_date)

Notice how you had to add the import statement to use the today() function. You can use functions from other modules in your class definition, but you need to import the module first, and the import statement has to be outside class definition.

# For use of np.sqrt
import numpy as np

class Point:
    """ A point on a 2D plane
    
   Attributes
    ----------
    x : float, default 0.0. The x coordinate of the point        
    y : float, default 0.0. The y coordinate of the point
    """
    def __init__(self, x=0.0, y=0.0):
      self.x = x
      self.y = y
      
    def distance_to_origin(self):
      """Calculate distance from the point to the origin (0,0)"""
      return np.sqrt(self.x ** 2 + self.y ** 2)
    
    def reflect(self, axis):
      """Reflect the point with respect to x or y axis."""
      if axis == "x":
        self.y = - self.y
      elif axis == "y":
        self.x = - self.x
      else:
        print("The argument axis only accepts values 'x' and 'y'!")

Notice how you implemented distance_to_origin() as a method instead of an attribute. Implementing it as an attribute would be less sustainable - you would have to recalculate it every time you change the values of the x and y attributes to make sure the object

Class attributes store data that is shared among all the class instances. They are assigned values in the class body, and are referred to using the ClassName. syntax rather than self. syntax when used in methods.

class Player:
    MAX_POSITION = 10
    
    def __init__(self):
        self.position = 0

    # Add a move() method with steps parameter
    def move(self, steps):
        if self.position + steps < Player.MAX_POSITION:       
            self.position = self.position + steps
        else:
            self.position = Player.MAX_POSITION    
    

       
    # This method provides a rudimentary visualization in the console    
    def draw(self):
        drawing = "-" * self.position + "|" +"-"*(Player.MAX_POSITION - self.position)
        print(drawing)

p = Player(); p.draw()
p.move(4); p.draw()
p.move(5); p.draw()
p.move(3); p.draw()

But it makes sense, when you think about it! You shouldn't be able to change the data in all the instances of the class through a single instance. Imagine if you could change the time on all the computers in the world by changing the time on your own computer! If you want to change the value of the class attribute at runtime, you need to do it by referring to the class name, not through an instance.

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