6.8. Polymorphism¶
6.8.1. Switch¶
It all starts with single if statement
language = 'English'
if language == 'Polish':
result = 'Witaj'
elif language == 'English':
result = 'Hello'
print(result)
# Hello
It quickly grows into multiple elif:
language = 'English'
if language == 'Polish':
result = 'Witaj'
elif language == 'English':
result = 'Hello'
elif language == 'Russian':
result = 'Привет'
else:
result = 'Unknown language'
print(result)
# Hello
In other languages you may find switch statement:
(note that this is not a valid Python code)
switch(language):
case 'Polish':
result = 'Witaj'
case 'English':
result = 'Hello'
case 'Russian':
result = 'Привет'
default:
result = 'Unknown language'
It's a bit cleaner, but essentially the same.
Problem is that, switch moves business logic to the execution place:
SWITCH = {'Polish': 'Witaj',
'English': 'Hello',
'German': 'Guten Tag'}
language = 'English'
result = SWITCH.get(language, 'Unknown language')
print(result)
# Hello
def switch(key):
return {
'Polish': 'Witaj'
'English': 'Hello',
'Russian': 'Привет',
}.get(key, 'Unknown language')
switch('English')
# Hello
switch('Russian')
# Привет
6.8.2. Pattern Matching¶
Since Python 3.10: PEP 636 -- Structural Pattern Matching: Tutorial
>>> language = 'English'
>>>
>>> # doctest: +SKIP
... match language:
... case 'Polish':
... result = 'Witaj'
... case 'English':
... result = 'Hello'
... case 'Russian':
... result = 'Привет'
... case _:
... result = 'Unknown language'
>>>
>>> # doctest: +SKIP
... print(result)
Hello
>>> status = 418
>>>
>>> # doctest: +SKIP
... match status:
... case 400:
... result = 'Bad request'
... case 401 | 403 | 405:
... result = 'Not allowed'
... case 404:
... result = 'Not found'
... case 418:
... result = "I'm a teapot"
... case _:
... result = 'Unexpected status'
>>> request = 'GET /index.html HTTP/2.0'
>>>
>>> # doctest: +SKIP
... match request.split():
... case ['GET', uri, version]:
... server.get(uri)
... case ['POST', uri, version]:
... server.post(uri)
... case ['PUT', uri, version]:
... server.put(uri)
... case ['DELETE', uri, version]:
... server.delete(uri)
>>> class Hero:
... def action():
... return ['move', 'left', 20]
>>>
>>> # doctest: +SKIP
... match hero.action():
... case ['move', ('up'|'down'|'left'|'right') as direction, value]:
... hero.move(direction, value)
... case ['make_damage', value]:
... hero.make_damage(value)
... case ['take_damage', value]:
... hero.take_damage(value)
>>> from enum import Enum
>>>
>>> class Key(Enum):
... ESC = 27
... ARROW_LEFT = 37
... ARROW_UP = 38
... ARROW_RIGHT = 39
... ARROW_DOWN = 40
>>>
>>> # doctest: +SKIP
... match keyboard.on_key_press():
... case Key.ESC:
... game.quit()
... case Key.ARROW_LEFT:
... game.move_left()
... case Key.ARROW_UP:
... game.move_up()
... case Key.ARROW_RIGHT:
... game.move_right()
... case Key.ARROW_DOWN:
... game.move_down()
... case _:
... raise ValueError(f'Unrecognized key')
>>> from enum import Enum
>>>
>>> class Color(Enum):
... RED = 0
... BLUE = 1
... BLACK = 2
>>>
>>> # doctest: +SKIP
... match color:
... case Color.RED:
... print('Soviet')
... case Color.BLUE:
... print('Allies')
... case Color.BLACK:
... print('Axis')
>>> from enum import Enum
>>>
>>> class SpaceMan(Enum):
... NASA = 'Astronaut'
... ESA = 'Astronaut'
... ROSCOSMOS = 'Cosmonaut'
... CNSA = 'Taikonaut'
... ISRO = 'GaganYatri'
>>>
>>> # doctest: +SKIP
... match agency:
... case SpaceMan.NASA:
... print('USA')
... case SpaceMan.ESA:
... print('Europe')
... case SpaceMan.ROSCOSMOS:
... print('Russia')
... case SpaceMan.CNSA:
... print('China')
... case SpaceMan.ISRO:
... print('India')
6.8.3. Polymorphism¶
from abc import ABCMeta, abstractmethod
from dataclasses import dataclass
@dataclass
class Person(metaclass=ABCMeta):
name: str
@abstractmethod
def say_hello(self):
pass
class Astronaut(Person):
def say_hello(self):
return f'Hello {self.name}'
class Cosmonaut(Person):
def say_hello(self):
return f'Привет {self.name}'
def hello(crew: list[Person]) -> None:
for member in crew:
print(member.say_hello())
if __name__ == '__main__':
crew = [Astronaut('Mark Watney'),
Cosmonaut('Иван Иванович'),
Astronaut('Melissa Lewis'),
Cosmonaut('Jan Twardowski')]
hello(crew)
# Hello Mark Watney
# Привет Иван Иванович
# Hello Melissa Lewis
# Привет Jan Twardowski
In Python, due to the duck typing and dynamic nature of the language, the Interface or abstract class is not needed to do polymorphism:
from dataclasses import dataclass
@dataclass
class Astronaut:
name: str
def say_hello(self):
return f'Hello {self.name}'
@dataclass
class Cosmonaut:
name: str
def say_hello(self):
return f'Привет {self.name}!'
if __name__ == '__main__':
crew = [Astronaut('Mark Watney'),
Cosmonaut('Иван Иванович'),
Astronaut('Melissa Lewis'),
Cosmonaut('Jan Twardowski')]
for member in crew:
print(member.say_hello())
# Hello Mark Watney
# Привет Иван Иванович
# Hello Melissa Lewis
# Привет Jan Twardowski
6.8.4. Use Cases¶
UIElement:
from abc import ABCMeta, abstractmethod
class UIElement(metaclass=ABCMeta):
@abstractmethod
def draw(self):
pass
class Input(UIElement):
def draw(self):
print('Drawing input')
class Button(UIElement):
def draw(self):
print('Drawing button')
def draw(element: UIElement):
element.draw()
if __name__ == '__main__':
draw(Textarea())
draw(Button())
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)]
Dynamic factory:
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)]