Functions available for listing summary fields.

def make_summary_funcs(rows, ids):
    """Functions available for listing summary fields."""
    return {
        'len': len,
        'list': lambda *x: filter(None, list(x)),
        'max': max,
        'min': min,
        'rows': partial(summary_rows, rows, ids),
        'sum': sum,
        'trace': print_trace
    }