def _paint(art, positions, characters):
    """Paint characters into their positions in the art.

    Args:
        art: the 2D list of str representing the art.
        positions: list of (x, y) with same size as characters.
        characters: list of str with same size as positions.

    Returns:
        modified art - 2D list of str.
    """
    for position, character in zip(positions, characters):
        art[int(position[0])][int(position[1])] = character
    return art
