'Iterate over all columns in the worksheet'

@property
def columns(self):

    max_row = self.max_row
    min_row = 1
    if (not self._cells):
        return ((),)
    cols = []
    for col_idx in range(self.max_column):
        cells = self.get_squared_range((col_idx + 1), min_row, (col_idx + 1), max_row)
        col = chain.from_iterable(cells)
        cols.append(tuple(col))
    return tuple(cols)
