当我从命令行运行命令时，python doctest未运行测试

Question

我在文件./src/string.py内有以下代码

#!/usr/bin/env python3.7


def extract_upper(phrase):
    """
    Take sthe string and returns the only uppercase letters

    The following is a doctest
    >>> extract_upper("Hello There, BOB")
    ['H', 'T', 'B', 'O', '']
    """

    return list(filter(str.isupper, phrase))


def extract_lower(phrase):
    return list(filter(str.islower, phrase))

if __name__=='__main__':
    print(f'Hello from string')

print(f"__name__ in string.py {__name__}")

当我从cmdline运行以下命令时。

python3.7 -m doctest -v  src/helpers/string.py

这不会进行任何测试。我得到以下输出。

19 items had no tests:
    string
    string.Formatter
    string.Formatter._vformat
    string.Formatter.check_unused_args
    string.Formatter.convert_field
    string.Formatter.format
    string.Formatter.format_field
    string.Formatter.get_field
    string.Formatter.get_value
    string.Formatter.parse
    string.Formatter.vformat
    string.Template
    string.Template.__init__
    string.Template._invalid
    string.Template.safe_substitute
    string.Template.substitute
    string._TemplateMetaclass
    string._TemplateMetaclass.__init__
    string.capwords
0 tests in 19 items.
0 passed and 0 failed.
Test passed.

有人可以帮我解决我做错了什么或我想念什么