Formatting 3 integers (hours, mins, secs) to `00:00:00`?
The % formatting operator already supports 0-padding, just specify the width as 02. Also, you can pass a tuple to combine multiple numbers into the same string with a single format: "%02d:%02d:%02d" % (h, m, s).

In Python 3 the format string method is slightly preferred to the % operator, with Python 3.6 further allowing the format string to reference the variables directly. Using such formatted string literals or f-strings, the formatting would be expressed as: f"{h:02}:{m:02}:{s:02}".

As pointed out in the comments, if you're starting with datetime objects to begin with, you can use their own formatting functionality instead.