The Open Document Format for Office Applications (OpenDocument)v1.2 on which numpy-financial is based specifies two functions which are currently not implemented: CUMPRINC and CUMIPMT. These functions calculate the cumulative principal and interest paid down in an installment loan after a number of payment periods.

To implement these, one could simply callnpf.ppmt/npf.ipmt with a 1-D array for theper argument and then sum over rows.

I'd be happy to contribute code and submit a pull request.