即使已经在评论中解决了，这里的答案也可以帮助访问问题的人，更容易找到解决方案.

Even if it was already solved in the comment, here the answer to help people visiting the question, finding the solution easier.

问题是文档测试块的缩进:

The problem was the indent of the block of the doctest:

def my_func():
    '''
    Dummy test function. Returns the number 5.
    .. doctest::

        >>> my_func()
        5
    '''
    return 5

只有在 doctest 块之后缩进的行才被视为测试的一部分并进行测试.

only the lines that are indented after the doctest block are considered part of the test and are tested.