'Return an iterator of the values with replacements based on fill_values'

def fill_values(self, col, col_str_iters):

    is_masked_column = hasattr(col, 'mask')
    has_fill_values = hasattr(col, 'fill_values')
    for (idx, col_str) in enumerate(col_str_iters):
        if (is_masked_column and has_fill_values):
            if col.mask[idx]:
                (yield col.fill_values[core.masked])
                continue
        if has_fill_values:
            if (col_str in col.fill_values):
                (yield col.fill_values[col_str])
                continue
        (yield col_str)
