TypeVar คลาสทั่วไป และ Protocol

TypeVar คืออะไร

from typing import TypeVar

T = TypeVar("T")

def identity(value: T) -> T:
    return value

print(identity(42))      # int
print(identity("hello")) # str

ฟังก์ชันแบบเจเนอริก

from typing import TypeVar

T = TypeVar("T")

def first(items: list[T]) -> T:
    return items[0]

x: int = first([1, 2, 3])     # mypy knows x is int
s: str = first(["a","b","c"]) # mypy knows s is str

TypeVar แบบมีข้อจำกัด

from typing import TypeVar

Numeric = TypeVar("Numeric", int, float)

def double(n: Numeric) -> Numeric:
    return n * 2

print(double(5))    # 10
print(double(2.5))  # 5.0
# double("x")  # mypy error

คลาสแบบเจเนอริก

from typing import Generic, TypeVar

T = TypeVar("T")

class Stack(Generic[T]):
    def __init__(self) -> None:
        self._items: list[T] = []

    def push(self, item: T) -> None:
        self._items.append(item)

    def pop(self) -> T:
        return self._items.pop()

stack: Stack[int] = Stack()
stack.push(1)
print(stack.pop())   # 1

TypeVar หลายตัว

from typing import Generic, TypeVar

K = TypeVar("K")
V = TypeVar("V")

class Pair(Generic[K, V]):
    def __init__(self, key: K, value: V) -> None:
        self.key = key
        self.value = value

p: Pair[str, int] = Pair("age", 30)
print(p.key, p.value)   # age 30

Protocol — การสืบชนิดแบบโครงสร้าง

from typing import Protocol

class Drawable(Protocol):
    def draw(self) -> None: ...

def render(obj: Drawable) -> None:
    obj.draw()

class Circle:
    def draw(self) -> None:
        print("Drawing circle")

render(Circle())   # works — Circle satisfies Drawable

Protocol พร้อม runtime_checkable

from typing import Protocol, runtime_checkable

@runtime_checkable
class Closeable(Protocol):
    def close(self) -> None: ...

class File:
    def close(self) -> None: print("closed")

print(isinstance(File(), Closeable))   # True

Protocol กับ ABC

from typing import Protocol
from abc import ABC, abstractmethod

# ABC: nominal typing
class Printable(ABC):
    @abstractmethod
    def display(self) -> None: ...

# Protocol: structural typing
class Displayable(Protocol):
    def display(self) -> None: ...

TypeVar พร้อม bound

from typing import TypeVar

class Animal:
    def speak(self) -> str: return "..."

A = TypeVar("A", bound=Animal)

def make_speak(animal: A) -> str:
    return animal.speak()

นามแฝงแบบเจเนอริก — Python 3.12

# Python 3.12+
type Vector = list[float]
type Matrix = list[Vector]

class Stack[T]:
    def __init__(self) -> None: self._items: list[T] = []
    def push(self, item: T) -> None: self._items.append(item)
    def pop(self) -> T: return self._items.pop()

ชนิด Self

from typing import Self

class Builder:
    def __init__(self) -> None:
        self.parts: list[str] = []

    def add(self, part: str) -> Self:
        self.parts.append(part)
        return self

b = Builder().add("a").add("b")
print(b.parts)   # ["a", "b"]

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