How to retrieve ETF constituents(holdings) weightings information by Bloomberg API?

Question

How to retrieve ETF constituents(holdings) weightings information by bloomberg API?

**xbbg or pdblp **

below is for Index and it's working

    import pdblp
    con = pdblp.Bcon()
    con.start()
    con.bulkref('BCOM Index','INDEX_MWEIGHT') 

but, not working ro ETF like 'IVV US Equity' .

enter image description here

the output information is what i want