python what does "%ds" mean in format?

Question

What does "%ds" in following codes mean?

theline = 'aaaaaeeebbbbbbbbccccccccddd'
baseformat = "5s 3x 8s 8s"
numremain = len(theline) - struct.calcsize(baseformat)
format = "%s %ds" % (baseformat, numremain)
s1, s2, s3, s4= struct.unpack(format, theline)

Answer 1

The '%ds' is not a format specifier. It's '%d' followed by 's':

>>> "%ds" % 6
6s