npf.rate() returns list of all nans where only one input PV/FV pair is problematic

[tylerdegroff]

Hello,

Today I noticed that where a list of present values and a list of future values are used as input to npf.rate(), each element in the list returned is nan if only just one present/future value combination exhibits the same sign. Is this behavior intentional? I would have expected only that particular problematic pair to have returned nan, as the other pairs' results remain valid and calculable.

As you can see by the reproducible example below, all you need to do to resolve this case is drop the last element from each list and the preceding elements (all of which exhibit opposing signs) calculate as expected. Leave the last elements in place, and you get back a list of all nan.

Thanks!
-Tyler

import numpy_financial as npf

pv = [-593.06, -4725.38, -662.05, -428.78, -13.65]
fv = [214.07, 4509.97, 224.11, 686.29, -329.67]

print(npf.rate(2, 0, pv, fv))

[nan nan nan nan nan]

print(npf.rate(2, 0, pv[0:-1], fv[0:-1]))

[-0.39920185 -0.02305873 -0.41818459 0.26513414]

[Kai-Striega]

Hi Tyler,

I've had a look around the code and it looks like it is done intentionally. Whether this should be the desired behavior is another question. I believe that in your example a better result would be the rate where it can be found and nan where it can't. If you agree with this I'd be happy to implement an update.

Thank you for raising this issue.

Kai

[tylerdegroff]

Hi Kai,

Thank you for taking a look at this question. And yes, I do agree that returning the rate where it can be found would be an improved result.

-Tyler