9.9. OOP Type Annotation¶
9.9.1. Rationale¶
All classes are types
Always depend upon abstraction not an implementation (SOLID Dependency Inversion Principle). More information in S.O.L.I.D.
class Astronaut:
pass
mark: Astronaut = Astronaut()
class Cache:
pass
class DatabaseCache(Cache):
pass
class MemoryCache(Cache):
pass
class FilesystemCache(Cache):
pass
db: Cache = DatabaseCache()
mem: Cache = MemoryCache()
fs: Cache = FilesystemCache()
9.9.2. Method Return Types¶
class Astronaut:
def say_hello(self) -> str:
return 'My name... José Jiménez'
mark: Astronaut = Astronaut()
mark.say_hello()
# 'My name... José Jiménez'
class Point:
def get_coordinates(self) -> Tuple[int, int]:
return 1, 2
pt: Point = Point()
pt.get_coordinates()
# (1, 2)
9.9.3. Required Method Arguments¶
class Point:
def __init__(self, x: int, y: int) -> None:
self.x = x
self.y = y
class Astronaut:
def __init__(self, firstname: str, lastname: str) -> None:
self.firstname: str = firstname
self.lastname: str = lastname
9.9.4. Optional Method Arguments¶
from typing import Tuple
class Point:
def __init__(self, x: int = 0, y: int = 0) -> None:
self.x = x
self.y = y
def set_coordinates(self, x: int, y: int) -> None:
self.x = x
self.y = y
def get_coordinates(self) -> Tuple[int, int]:
return self.x, self.y
pt: Point = Point()
pt.set_coordinates(1, 2)
pt.get_coordinates()
# (1, 2)
9.9.5. Classes¶
class Point:
def __init__(self, x: int = 0, y: int = 0) -> None:
self.x = x
self.y = y
def __str__(self) -> str:
return f'Point(x={self.x}, y={self.y})'
class Position:
def __init__(self, initial_position: Point = Point()) -> None:
self.position = initial_position
def get_coordinates(self) -> Point:
return self.position
pos: Position = Position()
pos.get_coordinates()
# <__main__.Point object at 0x11c5531c0>
print(pos.get_coordinates())
# Point(x=0, y=0)
9.9.6. Nested¶
from typing import List
class Iris:
def __init__(self, features: List[float], label: str) -> None:
self.features: List[float] = features
self.label: str = label
data: List[Iris] = [
Iris([4.7, 3.2, 1.3, 0.2], 'setosa'),
Iris([7.0, 3.2, 4.7, 1.4], 'versicolor'),
Iris([7.6, 3.0, 6.6, 2.1], 'virginica'),
]
9.9.7. Final Class¶
New in version Python: 3.8 See PEP 591
from typing import final
@final
class Astronaut:
pass
from typing import final
@final
class Astronaut:
pass
class Pilot(Astronaut):
pass
9.9.8. Final Method¶
New in version Python: 3.8 See PEP 591
from typing import final
class Astronaut:
@final
def say_hello(self) -> None:
pass
Listing 9.42. Error: Cannot override final attribute "foo" (previously declared in base class "Base")¶
from typing import final
class Astronaut:
@final
def say_hello(self) -> None:
pass
class Pilot(Astronaut):
def say_hello(self) -> None: # Error: Cannot override final attribute
pass
9.9.9. Final Attribute¶
from typing import Final
class Position:
x: Final[int]
y: Final[int]
def __init__(self) -> None:
self.x = 1
self.y = 2
from typing import Final
class Position:
x: Final[int]
y: Final[int] # Error: final attribute 'y' without an initializer
def __init__(self) -> None:
self.x = 1
from typing import Final
class Settings:
RESOLUTION_X_MIN: Final[int] = 0
RESOLUTION_X_MAX: Final[int] = 1024
RESOLUTION_Y_MIN: Final[int] = 0
RESOLUTION_Y_MAX: Final[int] = 768
class Game(Settings):
RESOLUTION_X_MIN = 3 # Error: can't override a final attribute
from typing import Final
class Hero:
DAMAGE_MIN: Final[int] = 10
DAMAGE_MAX: Final[int] = 20
Hero.DAMAGE_MIN = 1 # Error: can't override a final attribute
9.9.10. More Information¶
Note
More information in Type Annotations and Type Checking