OOP

Object-oriented programming

Everything in Python is an object!

>>> type(42)
<class 'int'>
>>> type([])
<class 'list'>
>>> type([]) is list
True
>>> type(list)
<class 'type'>
>>> type(list) is type
True
>>> type(type)
<class 'type'>

Why do we need OOP?

The four principles of OOP

 

  • Abstraction

  • Encapsulation

  • Inheritance

  • Polymorphism

Abstraction

Classes

class Vector:
    def __init__(self, x, y):
        self.x = x
        self.y = y

spam = Vector(1.0, 1.0)
  1.  __init__ is the constructor.
  2. The first argument accepted by the constructor is the instance itself.
  3. Class attributes don't need declaration.
  4. The class is instantiated by "calling" it with the arguments expected by the __init__ method.
class Panda:
    def __init__(self, name, age, weight):
        self.name = name
        self.age = age
        self.weight = weight

    def get_buffed(self):
        if self.weight < 1000:
            self.weight += 1

    def eat_bamboo(self):
        self.get_buffed()
        return "Nomm nomm nomm!"

    def celebrate_birthday(self):
        self.age += 1

Methods

Encapsulation

Private/protected

  • Classes in python are "open"

  • attributes/methods starting with _ are "protected"

  • attributes/methods starting with __ are "private". This is also called Name mangling and the Python interpreter does it itself.

class Panda:
    def __init__(self, name, age, weight):
        self.name = name
        self._age = age
        self.__weight = weight

    def _get_buffed(self):
        if self.__weight < 1000:
            self.__weight += 1

    def eat_bamboo(self):
        self._get_buffed()
        return "Nomm nomm nomm!"

    def celebrate_birthday(self):
        self._age += 1

Private/protected example

Bound/Unbound methods

import math

class Vector:
    def __init__(self, x, y):
        self.x = x
        self.y = y
    
    def length(self):
        return math.sqrt(self.x^2 + self.y^2)


v1 = Vector(2.0, 3.0)
v2 = Vector(1.0, 2.0)

print(Vector.length(v1))
print(Vector.length(v2))

Comparing objects

  • To compare the values of object use ==
  • To compare if two objects are the same use is
  • Override the method __eq__ to handle your own equalization
  • By default, __eq__ is implemented with is

Dunder methods

Known also as "magic methods",

"dunder"(double under) methods in Python override some aspects of the behavior of our objects.

  • __int__
  • __str__
  • __eq__
  • __repr__
  • __hash__

Dunder methods examples

class Panda:

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

    def _get_buff(self):
        if self.weight < 1000:
            self.weight += 1

    def eat_bamboo(self):
        self._get_buff()
        return "Nomm nomm nomm!"

    def __str__(self):
        return "I am a panda - {}".format(self.name)

getattr/setattr

v = Vector(4.0, 5.0)

print(getattr(v, 'y')) # 5.0

setattr(v, 'y', 10.0)
print(getattr(v, 'y')) # 10.0

OOP First Steps

By Hack Bulgaria

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OOP First Steps

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