12. Good Practices

12.1. Rationale

• Zasady clean code

• Nazewnictwo zmiennych, długość nazw, funkcji, klas i pakietów

• Kod po polsku czy angielsku?

• Liczba parametrów, typy i wzorce konstrukcyjne

• Zasady SOLID

• Zmienne statyczne, do czego służą i kiedy stosować

• Importy wszystkiego czy wybiórcze

• Klasy abstrakcyjne vs Interfejsy, kiedy i jak

• Wzorce projektowe

• Długość klas

• Liczba metod

• Długość metod

• Modyfikatory dostępu: private, protected, public

12.2. Komentarze

• Kiedy komentować

• Co komentować

• Zakomentowany kod

• TODO i FIXME

• JavaDoc w komentarzach

12.3. Code Language

• import this - The Zen of Python, by Tim Peters

• Readability counts.

• Special cases aren't special enough to break the rules.

• Although practicality beats purity.

• If the implementation is hard to explain, it's a bad idea.

12.4. Case Study - Administrative Divisions

• In US: The states are not administrative divisions of the country, in that their powers and responsibilities are in no way assigned to them from above by federal legislation or the Constitution; rather they exercise all powers of government not delegated to the federal government by the Constitution.

• Political divisions of the United States are the various recognized governing entities that together form the United States – states, the District of Columbia, territories and Indian reservations.

• https://en.wikipedia.org/wiki/Administrative_division

• https://en.wikipedia.org/wiki/List_of_administrative_divisions_by_country

• https://en.wikipedia.org/wiki/Administrative_division

class Obywatel:
    getWojewodztwo()
    getPowiat()
    getGmina()

class Citizen:
    getVoivodeship()
    getState()
    getCounty()
    getCeremonialCounty()
    getMetropolitanCounty()
    getDistrict()
    getCivilParish()

12.5. Identification Numbers

• https://github.com/arthurdejong/python-stdnum/tree/master/stdnum

class Obywatel:
    getPESEL()
    getNIP()

class Citizen:
    getSSN()
    getVATEU()

12.6. Tell - don't ask

• Tell-Don't-Ask is a principle that helps people remember that object-orientation is about bundling data with the functions that operate on that data.

• It reminds us that rather than asking an object for data and acting on that data, we should instead tell an object what to do.

• This encourages to move behavior into an object to go with the data.

Bad:

class Light:
    status = 'off'


light = Light()
light.status = 'on'
light.status = 'off'

Good:

class Light:
    status = 'off'

    def switch_on(self):
        self.status = 'on'

    def switch_off(self):
        self.status = 'off'


light = Light()
light.switch_on()
light.switch_off()

Bad:

class Hero:
    health: int = 10


hero = Hero()

while hero.health > 0:
    hero.health -= 2

Good:

class Hero:
    health: int = 10

    def is_alive(self):
        return self.health > 0

    def take_damage(self, damage):
        self.health -= damage


hero = Hero()

while hero.is_alive():
    hero.take_damage(2)

12.7. Setters, Getters, Deleters

• Java way: setters, getters, deleters

• Python way: properties, reflection, descriptors

• More information in Protocol Property

• More information in Protocol Reflection

• More information in Protocol Descriptor

• In Python you prefer direct attribute access

Accessing class fields using setter and getter:

class Astronaut:
    _name: str

    def set_name(self, name):
        self._name = name

    def get_name(self):
        return self._name


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

Problem with setters and getters:

class Point:
    _x: int
    _y: int

    def get_x(self):
        return self._x

    def set_x(self, value):
        self._x = value

    def del_x(self):
        del self._x

    def get_y(self):
        return self._y

    def set_y(self, value):
        self._x = value

    def del_y(self):
        del self._y

Rationale for Setters and Getters:

class Temperature:
    kelvin: int

    def set_kelvin(self, kelvin):
        if kelvin < 0:
            raise ValueError('Kelvin cannot be negative')
        else:
            self._kelvin = kelvin


t = Temperature()
t.set_kelvin(-1)
# Traceback (most recent call last):
# ValueError: Kelvin cannot be negative

12.8. References

• https://python.astrotech.io/oop/good-practices.html

12.9. Assignments

• Razem z trenerem omówcie filmik Uncle Boba dotyczący hidden classes

• Razem z trenerem oglądnijcie filmik Ruby i JS (opcjonalnie)