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2.0 Python: Classes/Objects

Python is an object oriented programming language.

Almost everything in Python is an object, with its properties and methods.

A Class is like an object constructor, or a "blueprint" for creating objects.

2.1 Create a Class​

To create a class, use the keyword class:

2.1.1 Example​

Create a class named MyClass, with a property named x:

class MyClass:
    x = 5

2.2 Create Object​

Now we can use the class named MyClass to create objects:

2.2.1 Example​

Create an object named p1, and print the value of x:

class MyClass:
    x = 5

# Create the Object
p1 = MyClass()

print(p1.x)

2.3 The init() Method​

The examples above are classes and objects in their simplest form, and are not really useful in real life applications.

To understand the meaning of classes we have to understand the built-in __init__() method.

All classes have a method called __init__(), which is always executed when the class is being initiated.

Use the __init__() method to assign values to object properties, or other operations that are necessary to do when the object is being created:

2.3.1 Example​

Create a class named Person, use the __init__() method to assign values for name and age:

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

p1 = Person("John", 36)

print(f"{p1.name} {p1.age}")
note

The __init__() method is called automatically every time the class is being used to create a new object.

2.4 The str() Method​

The __str__() method controls what should be returned when the class object is represented as a string.

If the __str__() method is not set, the string representation of the object is returned:

2.4.1 Example​

The string representation of an object WITHOUT the __str__() method:

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

p1 = Person("John", 36)

print(p1)

Output: <__main__.Person object at 0x0000028D8EEEF230>

2.4.2 Example​

The string representation of an object WITH the __str__() method:

class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age
    
    def __str__(self):
        return f"{self.name}({self.age})"
    
p1 = Person("John", 36)

print(p1)

Output: John(36)

2.5 Create Methods​

You can create your own methods inside objects. Methods in objects are functions that belong to the object.

Let us create a method in the Person class:

2.5.1 Example​

Insert a function that prints a greeting, and execute it on the p1 object:

class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age
    
    def myfunc(self):
        print("Hello my name is " + self.name)

p1 = Person("John", 36)

p1.myfunc()
note

The self parameter is a reference to the current instance of the class, and is used to access variables that belong to the class.

2.6 The self Parameter​

The self parameter is a reference to the current instance of the class, and is used to access variables that belong to the class.

It does not have to be named self, you can call it whatever you like, but it has to be the first parameter of any function in the class:

2.6.1 Example​

Use the words mysillyobject and abc instead of self:

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

  def myfunc(abc):
    print("Hello my name is " + abc.name)

p1 = Person("John", 36)
p1.myfunc()

2.7 Modify Object Properties​

You can modify properties on objects like this:

2.7.1 Example​

Set the age of p1 to 40:

p1.age = 40
class Person:
  def __init__(mysillyobject, name, age):
    mysillyobject.name = name
    mysillyobject.age = age

  def myfunc(abc):
    print("Hello my name is " + abc.name)
  
  def __str__(self):
    return f"{self.name} ({self.age})"

p1 = Person("John", 36)
p1.age = 40

print(p1)
p1.myfunc()

2.8 Delete Object Properties​

You can delete properties on objects by using the del keyword:

2.8.1 Example​

Delete the age property from the p1 object:

del p1.age
class Person:
  def __init__(mysillyobject, name, age):
    mysillyobject.name = name
    mysillyobject.age = age

  def myfunc(abc):
    print("Hello my name is " + abc.name)
  
  def __str__(self):
    return f"{self.name} ({self.age})"

p1.age = 40
del p1.age
p1 = Person("John", 36)

print(p1)
p1.myfunc()

Output:
John (36)
Hello my name is John

2.9 The pass Statement​

class definitions cannot be empty, but if you for some reason have a class definition with no content, put in the pass statement to avoid getting an error.

2.9.1 Example​

class Person:
    pass