函数输出似乎是12，但是作为NoneType对象返回？

I have a function:

我有一个功能:

def check_disk_space(path):
    import os
    """
    check_disk_space() checks the available space of a specified path
    """
    cmdparts = ["echo $(df --output=pcent ", ") | tr -d 'Use% '"]
    check_disk_space_cmd =  cmdparts[0] + path + cmdparts[1]
    print type(os.system(check_disk_space_cmd))
    return os.system(check_disk_space_cmd)
def check_disk_s