Compute the payment against loan principal plus interest.
Parameters:  rate : array_like
nper : array_like
pv : array_like
fv : array_like
when : {{‘begin’, 1}, {‘end’, 0}}, {string, int}


Returns:  out : ndarray

Notes
The payment pmt is computed by solving the equation:
fv +
pv*(1 + rate)**nper +
pmt*(1 + rate*when)/rate*((1 + rate)**nper  1) == 0
or, when rate == 0:
fv + pv + pmt * nper == 0
Examples
What would the monthly payment need to be to pay off a $200,000 loan in 15 years at an annual interest rate of 7.5%?
>>> np.pmt(0.075/12, 12*15, 200000)
1854.0247200054619
In order to payoff (i.e. have a futurevalue of 0) the $200,000 obtained today, a monthly payment of $1,854.02 would be required.