Python Classes

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#oop #classes #programming #TensorFlow #data

Classes are objects, and in Python, every object has a:

• Type
• Internal data representation (a blueprint)
• Methods (a set of procedures for interacting with or manipulating the object)

==An object is an instance of a particular type.== <-- It sounds like they're using class/type interchangebly.

This is where the type() function comes into play. It returns what ==type== or ==class== something is.

note

A class does not need to have a constructor, and classes built without one can still have class variables and methods, but cannot take arguments to set those variable's values at initialisation.

A class's methods are functions the every instance of that class's objects provide. It's how you interact with the data in the object.

For example, the elements in a list or the key-value pairs in a dictionary.

sc_members.sort() <-- sort() is a method of the class ==list== to which sc_members belongs.

Defining your own class

The structure of a class:

Syntax

class Name_of_class(parent_class ):

init()

To create your class, you must initialise its data and method structure first. This includes declaring the Class as above and initialising its data blueprint with the __init__() function.

The anatomy of a the __init__() class constructor is especially important, and other 'dunder' functions exist to create more complex classes. This one is required for all classes.

Attributes

An example: Creating a class for a circle and a rectangle.

Class Circle:

class Circle(object ):
	def __init__(self, radius, colour):
		self.radius = radius
		self.colour = colour

• Data Attributes
  • Radius
  • Colour

Class Rectangle

class Rectangle(object ):
	def __init__(self, height, width, colour):
		self.height = height
		self.width = width
		self.colour = colour

• Data Attributes
  • Height
  • Width
  • Colour

Creating an instance of a class:

Creating an instance of the class is as simple as calling the class initialiser function, but in place of __init__() you use the name of the class.

instance = classname(attribute1, attribute2)

E.g.

myCirc = Circle(12, "red")

Accessing class self parameters/attributes

Say you want to know what the circle's radius or colour is. You simply use ==dot notation==

---> print(myCirc.radius) ---> 12 ---> print(myCirc.colour) ---> "red"

Changing attributes

You can set the value of an object's attribute with the assignment operator = directly.

myCirc.radius = 12

myRec.colour = 'racing_green'

Class Variables Vs Instance Variables

Classes can have two types of variables, class variables and instance variables.

• Class variables or private variables are those that have values inherent to the class, such as a car class having 4 wheels or having a vehicle legal type.
• Instance variables or public variables are variables specific to that instance, such as a car class being a Mercedes make and a C30 model.

Class variables are declared straight away beneath the docstring while instance variables are created in the initialisation function, __init__(). See the syntax below.

Class CarObject(object ):
"""
A representation of a car
"""
	wheels = 4
	chassis = 1
	engines = 1
	legal_type = "vehicle"
	
	def __init__(self, make, model, registration)
		self.make = make
		self.model = model
		self.registration = registration

my_car = CarObject("Mercedes", "C30", "FARQ2")

# Accessing a Class variable

CarObject.engines = 2

# Accessing an Instance variable

my_car.registration = "TK421"

# but also ...

my_car.legal_type = "vehicle"

# though not ...

CarObject.make <-- This would throw an error

Methods

Methods are functions built into an object. They manipulate, populate, and help interact with the object.

Syntax

class Circle(object ):
	def __init__(self, radius, colour):
		self.radius = radius
		self.colour = colour
	
	def add_radius(self, r):
		self.radius = self.radius + r
	
	def reduce_radius(self, r):
		if self.radius - r > 0:
			self.radius = self.radius - r
		else:
			print("Circles must have a positive radius.")
	
	def drawCircle(self):
		pass

myCirc = Circle(12, "racing_green"

myCirc.radius --> 12 myCirc.colour --> "racing_green" mycirc.drawCircle()

pie
	title myCirc 
	"radius" : 12

The 'dir()' function

The dir() function, when passed the name of an object, will show you the attributes and methods of the object.