def nonexisting_file_checksum():
assert checksum('nosuchfile', 10) is None
def existing_file_checksum():
path = os.path.join(os.getcwd(), 'requirements.dev.txt')
must_be = '0cef80c690b7529d44b332fc'
h = checksum(path, 24)
assert h == must_be
assert len(h) <= 24