Python怎么去写单元测试用例去测试hello world呢

逛着博客园，看到乙醇大佬的一篇随笔 https://www.cnblogs.com/nbkhic/p/9370446.html，于是就在想怎么测试这句hello world

print('hello world')

想法是修改stdout的指向到一个io.StringIO流中，然后把流中的数据与‘hello world’去比较，可是写完之后发现，程序虽然没报错，但是流中无数据写入，百思不得其解；只好换成文件流，代码如下：

import sys


def hi():
    print('hello world')

if __name__ == '__main__':
    old = sys.stdout

    with open('test','w') as oFile:
        sys.stdout = oFile
        hi()

    sys.stdout = old

    with open('test','r') as oFile:
        if 'hello world' + '
' == oFile.readline():
            print('PASS')
        else:
            print('FAIL',file=sys.stderr)

结果也输出了PASS

C:UserssuneeeAppDataLocalProgramsPythonPython36python.exe E:/wangjz/PyWorkSpace/LearnPython/test.py
PASS

Process finished with exit code 0

PS:至于比较字符串‘hello world’后面为什么要加个' '，可以看下print函数说明

def print(*args, sep=' ', end='
', file=None): # known special case of print
    """
    print(value, ..., sep=' ', end='
', file=sys.stdout, flush=False)
    
    Prints the values to a stream, or to sys.stdout by default.
    Optional keyword arguments:
    file:  a file-like object (stream); defaults to the current sys.stdout.
    sep:   string inserted between values, default a space.
    end:   string appended after the last value, default a newline.
    flush: whether to forcibly flush the stream.
    """
    pass