因为 NOSET 日志级别的效果是这样的：

 Logger.setLevel(lvl)

    Sets the threshold for this logger to lvl. Logging messages which are less severe than lvl will be ignored. When a logger is created, the level is set to NOTSET (which causes all messages to be processed when the logger is the root logger, or delegation to the parent when the logger is a non-root logger). Note that the root logger is created with level WARNING.

    The term ‘delegation to the parent’ means that if a logger has a level of NOTSET, its chain of ancestor loggers is traversed until either an ancestor with a level other than NOTSET is found, or the root is reached.

    If an ancestor is found with a level other than NOTSET, then that ancestor’s level is treated as the effective level of the logger where the ancestor search began, and is used to determine how a logging event is handled.

    If the root is reached, and it has a level of NOTSET, then all messages will be processed. Otherwise, the root’s level will be used as the effective level.

因为 logging.getLogger('')(root logger)的默认日志级别是 WARNING 不是 NOSET，所以 rotate_file_handler 将使用这个 WARNING 的日志级别。

将 logging.getLogger('') 的日志级别也改为 NOTSET 就可以了:

logger = logging.getLogger('')
logger.setLevel(logging.NOTSET)
logger.addHandler(rotate_file_handler)

$ cat log
2017-02-10 23:04:23,583 test.pyline:20 This is debug message
2017-02-10 23:04:23,584 test.pyline:21 This is info message
2017-02-10 23:04:23,584 test.pyline:22 This is warning message
2017-02-10 23:04:23,584 test.pyline:23 error

参考:

• https://docs.python.org/3/lib...