4.1. OOP Principles

4.1.1. Rationale

  • Encapsulation

  • Abstraction

  • Inheritance

  • Polymorphism

4.1.2. Encapsulation

  • Do not allow to modify attributes directly

  • Hide them behind setters and getters

  • Prevents objects to get to an invalid state

class KelvinTemperature:
    _value: float

    def set_value(self, value: float) -> None:
        if value >= 0:
            self._value = value

    def get_value(self) -> float:
        return self._value


t = KelvinTemperature()
t.set_value(1)
t.set_value(0)
t.set_value(-1)
t.get_value()

class Astronaut:
    _firstname: str
    _lastname: str

    def set_name(self, name):
        fname, lname = name.split()
        self._firstname = fname
        self._lastname = lname

    def get_name(self):
        return f'{self._firstname} {self._lastname}'

astro = Astronaut()
astro.set_name('Mark Watney')
astro.get_name()

4.1.3. Abstraction

  • Reduce complexity by hiding unnecessary details

  • User do not need what does it mean to send email, that you have to connect, auth and later disconnect

class MailService:
    def send_email(sender, rcpt, subject, body):
        self._connect()
        self._authenticate()
        self._disconnect()

    def _connect(self, timeout=1):
        print('Connect')

    def _authenticate(self):
        print('Authenticate')

    def _disconnect(self):
        print('Disconnect')


if __name__ == '__main__':
    ms = MailService()
    ms.send_email(...)

4.1.4. Inheritance

class Person:
    _firstname: str
    _lastname: str

class Astronaut(Person):
    pass

4.1.5. Polymorphism

  • Ability of an object to take many forms

from abc import ABCMeta, abstractmethod


class UIElement(metaclass=ABCMeta):
    @abstractmethod
    def draw(self):
        pass

class TextBox(UIElement):
    def draw(self):
        print('Drawing text box')


class CheckBox(UIElement):
    def draw(self):
        print('Drawing check box')


def draw(element: UIElement):
    element.draw()


if __name__ == '__main__':
    draw(TextBox())
    draw(CheckBox())

class Cache:
    def set(self, name: str, value: str) -> None: pass
    def get(self, name: str) -> str: pass
    def is_valid(self, name: str) -> bool: pass

class DatabaseCache(Cache):
    pass

class MemoryCache(Cache):
    pass

class FilesystemCache(Cache):
    pass


def get(cache: Cache, key):
    if not cache.is_valid(key):
        cache.set(key, '...')
    return cache.get(key)


get(DatabaseCache(), 'name')
get(FilesystemCache(), 'name')
get(MemoryCache(), 'name')