def cell_key_val_gen(iterable, shape, topleft=(0, 0)):
    """Generator of row, col, value tuple from iterable of iterables

    it: Iterable of iterables
    \tMatrix that shall be mapped on target grid
    shape: Tuple of Integer
    \tShape of target grid
    topleft: 2-tuple of Integer
    \tTop left cell for insertion of it

    """

    top, left = topleft

    for __row, line in enumerate(iterable):
        row = top + __row
        if row >= shape[0]:
            break

        for __col, value in enumerate(line):
            col = left + __col
            if col >= shape[1]:
                break

            yield row, col, value