Return a string which represents a row of data cells.

def _build_row(padded_cells, colwidths, colaligns, rowfmt):
    "Return a string which represents a row of data cells."
    if not rowfmt:
        return None
    if hasattr(rowfmt, "__call__"):
        return rowfmt(padded_cells, colwidths, colaligns)
    else:
        return _build_simple_row(padded_cells, rowfmt)