可迭代对象、迭代器与生成器：让数据按需流动

class Countdown:
    def __init__(self, start: int) -> None:
        self.current = start

    def __iter__(self) -> "Countdown":
        return self

    def __next__(self) -> int:
        if self.current <= 0:
            raise StopIteration
        value = self.current
        self.current -= 1
        return value


counter = Countdown(3)
print(next(counter))
print(list(counter))

from collections.abc import Iterator
from pathlib import Path


def error_lines(path: Path) -> Iterator[tuple[int, str]]:
    with path.open(encoding="utf-8") as file:
        for line_number, line in enumerate(file, start=1):
            text = line.rstrip("\n")
            if "ERROR" in text:
                yield line_number, text


def main() -> None:
    path = Path("app.log")
    path.write_text(
        "INFO service started\nERROR database timeout\nINFO retry\n",
        encoding="utf-8",
    )

    for number, text in error_lines(path):
        print(f"{number}: {text}")


if __name__ == "__main__":
    main()

2: ERROR database timeout

numbers = (number * number for number in range(1_000_000))
first_three = [next(numbers) for _ in range(3)]
print(first_three)