what's the difference between "when='D' " and "when='midnight'" for TimedRotatingFileHandler?

There are two relevant parameters to TimedRotatingFileHandler: when, and interval. Most of the possible values for when, such as D for days, or H for hours, work in conjunction with interval—for example, if when='D', interval=7 were specified, then the log would be rotated every seven days. Similarly, if when='H', interval=12 were specified, then the log would be rotated every twelve hours, starting when the script is launched. when='midnight' is a special case, in that the interval parameter is not used, and the log is rolled over on a daily basis at midnight, regardless of the time when the script is launched (see the source here).