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."