Classes and Object Oriented Python

 

In Python, you define a class by using the class keyword followed by a name and a colon. Then you use .__init__() to declare which attributes each instance of the class should have:

# dog.py

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

In the body of .__init__(), there are two statements using the self variable:

1. self.name = name creates an attribute called name and assigns the value of the name parameter to it.
2. self.age = age creates an attribute called age and assigns the value of the age parameter to it.

To instantiate this Dog class, you need to provide values for name and age. If you don’t, then Python raises a TypeError:

>>> Dog()
Traceback (most recent call last):
  ...
TypeError: __init__() missing 2 required positional arguments: 'name' and 'age'

To pass arguments to the name and age parameters, put values into the parentheses after the class name:

>>> miles = Dog("Miles", 4)
>>> buddy = Dog("Buddy", 9)

When you instantiate the Dog class, Python creates a new instance of Dog and passes it to the first parameter of .__init__(). This essentially removes the self parameter, so you only need to worry about the name and age parameters.

What is the use of self in Python

When working with classes in Python, the term “self” refers to the instance of the class that is currently being used. It is customary to use “self” as the first parameter in instance methods of a class. Whenever you call a method of an object created from a class, the object is automatically passed as the first argument using the “self” parameter. This enables you to modify the object’s properties and execute tasks unique to that particular instance.

The __init()___ is similar to constructors in C++ or JAVA. When you instantiate the Dog class, Python creates a new instance of Dog and passes it to the first parameter of .__init__(). This essentially removes the self parameter, so you only need to worry about the name and age parameters.

Instance methods are functions that you define inside a class and can only call on an instance of that class. Just like .__init__(), an instance method always takes self as its first parameter.

# dog.py

class Dog:
    species = "Canis familiaris"

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

    # Instance method
    def description(self):
        return f"{self.name} is {self.age} years old"

    # Another instance method
    def speak(self, sound):
        return f"{self.name} says {sound}"

Creating object and calling the methods

>>> miles = Dog("Miles", 4)

>>> miles.description()
'Miles is 4 years old'

>>> miles.speak("Woof Woof")
'Miles says Woof Woof'

>>> miles.speak("Bow Wow")
'Miles says Bow Wow'

Inheritance

The Base class Dog can be inherited by the child classes as below:

# dog.py

# ...

class JackRussellTerrier(Dog):
    def speak(self, sound="Arf"):
        return f"{self.name} says {sound}"

# ...    

Child class objects can be created as

>>> miles = JackRussellTerrier("Miles", 4)
>>> miles.speak()
'Miles says Arf'

Instances of child classes inherit all of the attributes and methods of the parent class

Parent Class functionality extension

# dog.py

# ...

class JackRussellTerrier(Dog):
    def speak(self, sound="Arf"):
        return f"{self.name} says {sound}"

# ...    

Here, speak() is overrided in the derived class

You can access the parent class from inside a method of a child class by using super():

# dog.py

# ...

class JackRussellTerrier(Dog):
    def speak(self, sound="Arf"):
        return super().speak(sound)

# ...

When you call super().speak(sound) inside JackRussellTerrier, Python searches the parent class, Dog, for a .speak() method and calls it with the variable sound.

Source: https://realpython.com/python3-object-oriented-programming/#how-do-you-define-a-class-in-python

Garbage Collection in Python

Python’s memory allocation and deallocation method is automatic. The user does not have to preallocate or deallocate memory similar to using dynamic memory allocation in languages such as C or C++ variables declared in heap. 

Python automatically schedules garbage collection based upon a threshold of object allocations and object deallocations. When the number of allocations minus the number of deallocations is greater than the threshold number, the garbage collector is run.

The garbage collection can be invoked manually in the following way: 

# Importing gc module import gc   # Returns the number of # objects it has collected # and deallocated collected = gc.collect()   # Prints Garbage collector # as 0 object print("Garbage collector: collected",           "%d objects." % collected)