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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."

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