def _numbers_units(N):
    """
    >>> _numbers_units(45)
    '123456789012345678901234567890123456789012345'
    """
    lst = range(1, N + 1)
    return "".join(list(map(lambda i: str(i % 10), lst)))
>>> _numbers_units(45)
    '123456789012345678901234567890123456789012345'