def get_valid_phone_number(input):
    """
    Will strip out the non-digits and make sure it's 10 numbers long. None is
    returned for input that does not contain exactly 10 digits
    >>> get_valid_phone_number('123')
    None
    >>> get_valid_phone_number('1231231234')
    '1231231234'
    >>> get_valid_phone_number('12(31##2asd31asdf234')
    '1231231234'
    """
    input = ''.join([c for c in input if c.isdigit()])
    if len(input) == 10:
        return input
    else:
        return None