The os.path.commonprefix function basically returns the common initial characters (if any) shared by a sequence of strings, e.g.     os.path.commonprefix(("Python is great!", "Python is not bad", "Python helps")) # returns "Python " The following was wrong.  os.path.commonprefix uses min and max, not sort.  I withdraw it: I also suspect that this function could be made more efficient.  It sorts the sequence.  While sorting is very fast (thanks, Uncle Tim!) it seems a bit OTT in this case. Thoughts? Best wishes Rob Cliffe