def isISBN(isbn):   """Checks if the passed string is a valid ISBN number."""   if len(isbn) != 10 or not isbn[:9].isdigit():     return False   if not (isbn[9].isdigit() or isbn[9].lower() == "x"):     return False   tot = sum((10 - i) * int(c) for i, c in enumerate(isbn[:-1]))   checksum = (11 - tot % 11) % 11   if isbn[9] == 'X' or isbn[9] == 'x':     return checksum == 10   else:     return checksum == int(isbn[9]) ok = """031234161X 0525949488 076360013X 0671027360 0803612079     0307263118 0684856093 0767916565 0071392319 1400032806 0765305240""" for code in ok.split():   assert isISBN(code) bad = """0312341613 052594948X 0763600138 0671027364 080361207X 0307263110      0684856092 0767916567 0071392318 1400032801 0765305241 031234161      076530Y241 068485609Y""" for code in bad.split():   assert not isISBN(code) print "Tests of isISBN()passed."