1. Python 的类型系统¶

from typing import List, TypeVar

T = TypeVar('T')


def first_and_last(items: List[T]) -> T:
    return items[0]


result = first_and_last([1, 2, 3, 4])  # result: int

from typing import Type


class Animal:
    @classmethod
    def make_sound(cls):
        pass


def mimic(animal_class: Type[Animal]):  # animal_class is a class, not an instance
    animal_class.make_sound()


mimic(Animal)

from typing import Any, TypeGuard


def is_integer(value: Any) -> TypeGuard[int]:
    return isinstance(value, int)

from typing import List, Union, TypeGuard


def is_string_list(values: List[Union[int, str]]) -> TypeGuard[List[str]]:
    return all(isinstance(value, str) for value in values)


def process(values: List[Union[int, str]]):
    if is_string_list(values):
        # Within this block, 'values' is treated as List[str] by the type checker
        concatenated = " ".join(values)
        print(concatenated)
    else:
        # Here, 'values' is still List[Union[int, str]]
        print("List contains non-string values.")

from typing import Generic, TypeVar

T = TypeVar('T')


class Box(Generic[T]):
    def __init__(self, item: T):
        self.item = item


class Container(Generic[T]):
    def __init__(self, value: T):
        self.value = value


box_int = Box(5)  # box_int: Box[int], class Box(item: int)
box_str = Box("Hello")  # box_str: Box[str], class Box(item: str)

# This allows for type-safe operations on the container
int_container = Container[int](5)
str_container = Container[str]("Hello")