Or:

"{:{:s}{:d}s}".format(str(self.pcs), self.format_align, self.max_length)

You're trying to apply the string format specifier (the stuff after the first colon through the final "s", as expanded) to an object that's not always a string: sometimes it's None. So you need to use one of the two supported ways to convert it to a string. Either str() or !s.

str.format() is very much dependent on the types of its arguments: the format specifier needs to be understood by the object being formatted. Similarly, you couldn't pass in a datetime and expect that to work, either.