I am stuck in a simple if condition for the below code

 for invoice in invoice_objs:
     invoice_date = invoice.invoice_date.strftime('%Y-%m-%d')
     worksheet.write(row, 0, invoice_date)
     worksheet.write(row, 1, invoice.name)
     worksheet.write(row, 2, invoice.partner_id.name)
     worksheet.write(row, 3,  ', '.join(invoice.invoice_line_ids.mapped('product_id.name')))
   -->  worksheet.write(row, 4, ', '.join(invoice.invoice_line_ids.mapped('tax_ids.name')))
     worksheet.write(row, 5, ', '.join(str(i) for i in invoice.invoice_line_ids.mapped('price_subtotal')))
   -->  if invoice.invoice_line_ids.mapped('tax_ids.name') == 'Sales VAT':
         worksheet.write(row, 6, '15%')
     else:
         worksheet.write(row, 6, '0%')
     row += 1
     key = u'_'.join((invoice.partner_id.name, invoice.currency_id.name)).encode('utf-8')
     key = str(key, 'utf-8')

I just want to fetch the value of one excel field and use in if condition

If tax_ids.name == 'Sales VAT'

it should print 15%

else 

0%

but for all the lines its taking only the else condition i.e 0%