类型提示在Python 3.7+中返回字符串或错误的方法

Question

我有一个python脚本，其开头是在当前目录中搜索CSV文件或下载目录以进行一些处理的方法。如果未找到文件CSV文件，则该程序不应运行并退出并显示错误消息。

我在下面输入了一种临时方法的注释：

import glob
import os


def get_csv_filename() -> str:
    """ Returns first csv filename in current folder or Downloads folder """
    csv_filenames = glob.glob('*.csv')
    if csv_filenames:
        return csv_filenames[0]
    home = os.path.expanduser("~")
    csv_filenames = glob.glob(home + "/Downloads/*.csv")
    if csv_filenames:
        return csv_filenames[0]
    # if I don't use return, I also get problems with pylint
    return exit("Error: no file found, check the documentation for more info.")


def main() -> None:
    """ Reads a CSV and does some processing """
    filename = get_csv_filename()

如果我输入带有例如pytype我得到错误：

get_csv_filename: bad option in return type [bad-return-type]
  Expected: str
  Actually returned: None

您建议如何使该代码兼容？

Answer 1

This section可能会有所帮助。我没有安装

mypy或pytype进行尝试，但是也许可以使用：

from typing import NoReturn

def get_csv_filename() -> str:
    """ Returns first csv filename in current folder or Downloads folder """
    csv_filenames = glob.glob('*.csv')
    if csv_filenames:
        return csv_filenames[0]
    ...
    stop("Error: no file found, check the documentation for more info.")
    return ""

def stop(msg) -> NoReturn:
    exit(msg)

另一个选择是：
from typing import Union

def get_csv_filename() -> Union[None, str]:
...

Answer 2

实际上基于Steve Bremer响应的思想，我实际上是通过一种更简单的方法解决了该问题：

from typing import Optional
def get_csv_filename() -> Optional[str]:
    ...

实际上Optional[something]等效于Union[None, something]。