def num2col(col_int: int) -> str:
    """
    Convert an Excel column index to a string
    e.g. 1 == "A", 27 == "AA" e.t.c.
    """
    if not isinstance(col_int, int):
        raise ValueError("Invalid data type supplied. Must supply an integer")
    col_str = ""
    while col_int > 0:
        col_int, remainder = divmod(col_int - 1, 26)
        col_str = chr(65 + remainder) + col_str
    return col_str