Python Notes (7) - Introduction to Classes

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The note focuses on Classes, which are a crucial part of object-oriented programming (OOP). In this lesson, we’ll explain what classes are, why they’re important, and how to use them effectively.

Python notes of open courses @Codecademy.

Class Basics

Why Use Classes?

Python is an object-oriented programming (OOP) language, which means it manipulates programming constructs called objects.

You can think of an object as a single data structure that contains data as well as functions; functions of objects are called methods.

A class is just a way of organizing and producing objects with similar attributes and methods.

Class Syntax

• A basic class consists only of the class keyword, the name of the class, and the class from which the new class inherits in parentheses.

  class NewClass(object):

  • This gives them the powers and abilities of a Python object. By convention, user-defined Python class names start with a capital letter.
• __init__()
  
  • This function is required for classes, and it’s used to initialize the objects it creates.
  • It always takes at least one argument, self, that refers to the object being created.
• self
  
  • Python will use the first parameter that __init__() receives to refer to the object being created.
  • This is why it’s often called self, since this parameter gives the object being created its identity.
• Dot Notation
  
  • To assign a variable to the class (creating a member variable), we use dot notation, self.new_variable = new_variable.
  • We can access attributes of our objects using dot notation, such as self.attribute.

Example

By Now, We Have:

# Class definition
class Animal(object):
    """Makes cute animals."""
    # For initializing our instance objects
    def __init__(self, name, age, is_hungry):
        self.name = name
        self.age = age
        self.is_hungry = is_hungry

# Note that self is only used in the __init__() function definition;
# we don't need to pass it to our instance objects.

zebra = Animal("Jeffrey", 2, True)

print zebra.name, zebra.age, zebra.is_hungry

Member Variables and Functions

Class Scope

Another important aspect of Python classes is scope. The scope of a variable/function is the context in which it’s visible to the program.

Not all variables are accessible to all parts of a Python program at all times. When dealing with classes, you can have variables/functions that are:

• available everywhere (global variables),
• only available to members of a certain class (member variables)
• only available to particular instances of a class (instance variables).

Inheritance

Inheritance is the process by which one class takes on the attributes and methods of another, and it’s used to express an is-a relationship. For example, a Panda is a bear, so a Panda class could inherit from a Bear class.

Inheritance Syntax

• class DerivedClass(BaseClass):
  
  • where DerivedClass is the new class you’re making and BaseClass is the class from which that new class inherits.

• Override

  • Sometimes you’ll want one class that inherits from another to not only take on the methods and attributes of its parent, but to override one or more of them.

  • Rather than have a separate greet_underling method for our CEO, we override (or re-create) the greet method on top of the base Employee.greet method. This way, we don’t need to know what type of Employee we have before we greet another Employee.

    class Employee(object):
    def __init__(self, name):
    self.name = name
    def greet(self, other):
    print "Hello, %s" % other.name
    
    class CEO(Employee):
        def greet(self, other):
            print "Get back to work, %s!" % other.name
    
    ceo = CEO("Emily")
    emp = Employee("Steve")
    emp.greet(ceo)
    
    # Hello, Emily
    
    ceo.greet(emp)
    
    # Get back to work, Steve!
    

• super call

  • In the flip side, sometimes you’ll be working with a derived class (or subclass) and realize that you’ve overwritten a method or attribute defined in that class’ base class (also called a parent or superclass) that you actually need. Have no fear! You can directly access the attributes or methods of a superclass with Python’s built-in super call.

  • The syntax looks like this:

    class Derived(Base):
        def m(self):
            return super(Derived, self).m()

    Where m() is a method from the base class.

• __repr__()

  • It is short for representation; by providing a return value in this method, we can tell Python how to represent an object of our class (for instance, when using a print statement).

    class Point3D(object):
    def __init__(self, x, y, z):
    self.x = x
    self.y = y
    self.z = z
    
        def __repr__(self):
            return "(%d, %d, %d)" % (self.x, self.y, self.z)
    
    my_point = Point3D(1,2,3)
    print my_point

Example

By Now, We Have A Comprehensive Example:

# 1. Creating a class
class Car(object):
    # 3. Creating member variables
    condition = "new"
    # 5. Initializing a class
    def __init__(self, model, color, mpg):
        self.model = model
        self.color = color
        self.mpg   = mpg

    # 6. Creating class methods
    def display_car(self):
        # 7. Referring to member variables
        return "This is a " + self.color + " " + self.model + " with "+ str(self.mpg) + " MPG."

    def drive_car(self):
        # 8. Modifying member variables
        self.condition = "used"
        return self.condition  

# 9. Inheritance
class ElectricCar(Car):
    def __init__(self, model, color, mpg, battery_type):
        self.model = model
        self.color = color
        self.mpg   = mpg
        self.battery_type = battery_type
    # 10. Overriding methods
    def drive_car(self):
        self.condition = "like new"
        return self.condition 

# 2. Creating an instance of a class
my_car = Car("DeLorean", "silver", 88)
# 4. Calling class member variables
print my_car.condition
my_car.drive_car()
print my_car.condition

my_car = ElectricCar("DeLorean", "silver", 88, "molten salt")
print my_car.condition
my_car.drive_car()
print my_car.condition

External Resources

• Object Oriented Programming Definition Read
• Modules, Classes, and Objects