def default_col_widths(dtype, col_header):
    """
    Assigns default column widths depending on the dtype. 
    Used in _str_ representation.
    """
    col_widths = []
    values = list(zip(*list(dtype.fields.values())))[0]
    for t in values:
        if t.kind == 'S':
            col_widths.append(t.itemsize + 5)
        else:
            col_widths.append(len(col_header) + 5)
    return col_widths
