4.8. Polymorphism

4.8.1. Switch

Switch moves business logic to the execution place:

watney = 'Astronaut'

if watney == 'Astronaut':
    print('Hello')
elif watney == 'Cosmonaut':
    print('Привет!')
elif watney == 'Taikonaut':
    print('你好')
else:
    print('Default Value')

# Hello

def say_hello(key=None):
    return {
        'Astronaut': 'Hello',
        'Cosmonaut': 'Привет!',
        'Taikonaut': '你好',
    }.get(key, 'Default Value')


watney = 'Astronaut'
ivanovic = 'Cosmonaut'
twardowski = 'Sorcerer'

say_hello(watney)
# Hello

say_hello(ivanovic)
# Привет!

say_hello(twardowski):
# 'Default Value'

4.8.2. Polymorphism in a Function

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 Sorcerer:
    pass

class Astronaut:
    def say_hello(self):
        return 'Hello'

class Cosmonaut:
    def say_hello(self):
        return 'Привет!'


def say_hello(spaceman):
    if hasattr(spaceman, 'say_hello')
        return spaceman.say_hello()
    else:
        return 'Default Value'


watney = Astronaut()
ivanovic = Cosmonaut()
twardowski = Sorcerer()

say_hello(watney)
# Hello

say_hello(ivanovic)
# Привет!

say_hello(twardowski)
# 'Default Value'

4.8.3. Polymorphism on a Class

Polymorphism on Classes:

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

    def say_hello(self):
        return 'Hello'


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

    def say_hello(self):
        return 'Привет!'


crew = [
    Astronaut('Mark Watney'),
    Cosmonaut('Иван Иванович'),
    Astronaut('Matt Kowalski'),
    Cosmonaut('Jan Twardowski'),
]

for member in crew:
    print(member.say_hello())
# Hello
# Привет!
# Hello
# Привет!

4.8.4. Factory

DATA = [('Sepal length', 'Sepal width', 'Petal length', 'Petal width', 'Species'),
        (5.8, 2.7, 5.1, 1.9, 'virginica'),
        (5.1, 3.5, 1.4, 0.2, 'setosa'),
        (5.7, 2.8, 4.1, 1.3, 'versicolor'),
        (6.3, 2.9, 5.6, 1.8, 'virginica'),
        (6.4, 3.2, 4.5, 1.5, 'versicolor'),
        (4.7, 3.2, 1.3, 0.2, 'setosa')]


class Iris:
    def __init__(self, sepal_length, sepal_width, petal_length, petal_width):
        self.sepal_length = sepal_length
        self.sepal_width = sepal_width
        self.petal_length = petal_length
        self.petal_width = petal_width

    def __repr__(self):
        name = self.__class__.__name__
        values = tuple(self.__dict__.values())
        return f'\n {name}{values}'


class Setosa(Iris):
    pass

class Virginica(Iris):
    pass

class Versicolor(Iris):
    pass


def factory(species: str):
    if species == 'setosa':
        return Setosa
    if species == 'virginica':
        return Virginica
    if species == 'versicolor':
        return Versicolor


result = []

for *features, species in DATA[1:]:
    iris = factory(species)
    i = iris(*features)
    result.append(i)

print(result)
# [Virginica(5.8, 2.7, 5.1, 1.9),
#  Setosa(5.1, 3.5, 1.4, 0.2),
#  Versicolor(5.7, 2.8, 4.1, 1.3),
#  Virginica(6.3, 2.9, 5.6, 1.8),
#  Versicolor(6.4, 3.2, 4.5, 1.5),
#  Setosa(4.7, 3.2, 1.3, 0.2)]

from dataclasses import dataclass

DATA = [('Sepal length', 'Sepal width', 'Petal length', 'Petal width', 'Species'),
        (5.8, 2.7, 5.1, 1.9, 'virginica'),
        (5.1, 3.5, 1.4, 0.2, 'setosa'),
        (5.7, 2.8, 4.1, 1.3, 'versicolor'),
        (6.3, 2.9, 5.6, 1.8, 'virginica'),
        (6.4, 3.2, 4.5, 1.5, 'versicolor'),
        (4.7, 3.2, 1.3, 0.2, 'setosa')]


@dataclass
class Iris:
    sepal_length: float
    sepal_width: float
    petal_length: float
    petal_width: float

class Setosa(Iris):
    pass

class Virginica(Iris):
    pass

class Versicolor(Iris):
    pass


def factory(species: str):
    species = species.capitalize()
    classes = globals()
    return classes[species]


result = [
    factory(species)(*features)
    for *features, species in DATA[1:]
]

print(result)
# [Virginica(sepal_length=5.8, sepal_width=2.7, petal_length=5.1, petal_width=1.9),
#  Setosa(sepal_length=5.1, sepal_width=3.5, petal_length=1.4, petal_width=0.2),
#  Versicolor(sepal_length=5.7, sepal_width=2.8, petal_length=4.1, petal_width=1.3),
#  Virginica(sepal_length=6.3, sepal_width=2.9, petal_length=5.6, petal_width=1.8),
#  Versicolor(sepal_length=6.4, sepal_width=3.2, petal_length=4.5, petal_width=1.5),
#  Setosa(sepal_length=4.7, sepal_width=3.2, petal_length=1.3, petal_width=0.2)]

4.8.5. Assignments