Classes and OOP I

The init Method

In the previous lesson you added attributes by hand after creating an object. The __init__ method lets you set them up automatically every time a new instance is created.

In the previous lesson you attached attributes to an object after creating it. That works, but it is easy to forget one. Python has a better way.

The __init__ method runs automatically every time you create a new instance:

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

Python

The first parameter is always self. It refers to the instance being created. You do not pass it yourself. Python handles that.

When you call Dog("Rex"), Python creates the object and passes it as self:

class Dog:
	def __init__(self, name):
		self.name = name
 
rex = Dog("Rex")
print(rex.name)

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Output

What will be the output?

class Cat:
	def __init__(self, color):
		self.color = color
 
c = Cat("black")
print(c.color)

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You can have multiple parameters, just like regular functions. Remember parameters from the Functions I series? Same rules apply here.

A class with two parameters in __init__:

class Car:
	def __init__(self, make, year):
		self.make = make
		self.year = year
 
car = Car("Toyota", 2020)
print(car.make)
print(car.year)

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Output

What will be the output?

class Point:
	def __init__(self, x, y):
		self.x = x
		self.y = y
 
p = Point(3, 7)
print(p.x + p.y)

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Default parameter values work inside __init__ too. You learned this pattern in Functions I.

If no color is passed, the default is used:

class Shirt:
	def __init__(self, size, color="white"):
		self.size = size
		self.color = color
 
s = Shirt("M")
print(s.color)

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Output

What will be the output?

class Lamp:
	def __init__(self, watts=60):
		self.watts = watts
 
a = Lamp()
b = Lamp(100)
print(a.watts, b.watts)

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You can also compute new attributes from the parameters. The body of __init__ is regular Python code.

This Rectangle computes its area during creation:

class Rectangle:
	def __init__(self, w, h):
		self.width = w
		self.height = h
		self.area = w * h
 
r = Rectangle(5, 3)
print(r.area)

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Output

What will be the output?

class Circle:
	def __init__(self, radius):
		self.r = radius
		self.d = radius * 2
 
c = Circle(4)
print(c.d)

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What will be the output?

class User:
	def __init__(self, first, last):
		self.full = first + " " + last
 
u = User("Ada", "Lovelace")
print(u.full)

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What will be the output?

class Pair:
	def __init__(self, a, b):
		self.total = a + b
		self.diff = a - b
 
p = Pair(10, 3)
print(p.total, p.diff)

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The __init__ Method

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The init Method