class UtilException(Exception):
    pass


def bulk_replace(filepath, pattern, replace_with, line=None):
    """
    Function which open file, find necessary pattern and replace it with new fragment.
    :param filepath: Path to file
    :param pattern: String to replace
    :param replace_with: The new string to replace with
    :param line: In which line in the file to replace (if empty, then replace all)
    :return:
    """
    try:
        assert pattern != replace_with
    except AssertionError:
        raise UtilException('pattern and replace_with can not be same')

    with open(filepath, 'r') as f:
        text = f.readlines()
        f.close()
        if line:  # assume we get line number, not index
            try:
                assert line >= 1
                try:
                    # replace necessary text
                    text[line-1] = text[line-1].replace(pattern, replace_with)
                    _write_lines(filepath, text)
                except IndexError:
                    raise UtilException('line not found')
            except AssertionError:
                raise UtilException('line index should be gte 1')
        else:
            # walk through all lines and replace necessary text
            lines = [line.replace(pattern, replace_with) for line in text]
            _write_lines(filepath, lines)


def _write_lines(filepath, lines):
    """
    Helper function
    :param filepath: File path to open
    :param lines: list of lines
    :return:
    """
    with open(filepath, 'w') as f:
        f.writelines(lines)
        f.close()