def read_file_to_list(filename):
    """ Reads a file containing digits(or a big number) and converts it to a
        list of individual.
    """
    digits = []
    with open(filename) as _file:
        while True:
            _char = _file.read(1)
            if not _char:  # End of file
                break
            else:
                try:
                    digits.append(int(_char))
                except ValueError:
                    pass
    return digits