Merge pull request #21 from person142/nper

BUG: fix issues with `nper`

Author: Kai-Striega

numpy_financial/_financial.py

@@ -281,21 +281,25 @@ def nper(rate, pmt, pv, fv=0, when='end'):
 
     """
     when = _convert_when(when)
-    (rate, pmt, pv, fv, when) = map(np.asarray, [rate, pmt, pv, fv, when])
+    rate, pmt, pv, fv, when = np.broadcast_arrays(rate, pmt, pv, fv, when)
+    nper_array = np.empty_like(rate, dtype=np.float64)
 
-    use_zero_rate = False
-    with np.errstate(divide="raise"):
-        try:
-            z = pmt*(1+rate*when)/rate
-        except FloatingPointError:
-            use_zero_rate = True
+    zero = rate == 0
+    nonzero = ~zero
 
-    if use_zero_rate:
-        return (-fv + pv) / pmt
-    else:
-        A = -(fv + pv)/(pmt+0)
-        B = np.log((-fv+z) / (pv+z))/np.log(1+rate)
-        return np.where(rate == 0, A, B)
+    with np.errstate(divide='ignore'):
+        # Infinite numbers of payments are okay, so ignore the
+        # potential divide by zero.
+        nper_array[zero] = -(fv[zero] + pv[zero]) / pmt[zero]
+
+    nonzero_rate = rate[nonzero]
+    z = pmt[nonzero] * (1 + nonzero_rate * when[nonzero]) / nonzero_rate
+    nper_array[nonzero] = (
+        np.log((-fv[nonzero] + z) / (pv[nonzero] + z))
+        / np.log(1 + nonzero_rate)
+    )
+
+    return nper_array
 
 
 def ipmt(rate, per, nper, pv, fv=0, when='end'):

numpy_financial/tests/test_financial.py

@@ -141,14 +141,6 @@ def test_ipmt_decimal(self):
         result = npf.ipmt(Decimal('0.1') / Decimal('12'), 1, 24, 2000)
         assert_equal(result.flat[0], Decimal('-16.66666666666666666666666667'))
 
-    def test_nper(self):
-        assert_almost_equal(npf.nper(0.075, -2000, 0, 100000.),
-                            21.54, 2)
-
-    def test_nper2(self):
-        assert_almost_equal(npf.nper(0.0, -2000, 0, 100000.),
-                            50.0, 1)
-
     def test_npv(self):
         assert_almost_equal(
             npf.npv(0.05, [-15000, 1500, 2500, 3500, 4500, 6000]),
@@ -410,3 +402,27 @@ def test_broadcast_decimal(self):
                             [Decimal('-74.998201'), Decimal('-75.62318601'),
                              Decimal('-75.62318601'), Decimal('-76.88882405'),
                              Decimal('-76.88882405')], 4)
+
+
+class TestNper:
+    def test_basic_values(self):
+        assert_allclose(
+            npf.nper([0, 0.075], -2000, 0, 100000),
+            [50, 21.544944],  # Computed using Google Sheet's NPER
+            rtol=1e-5,
+        )
+
+    def test_gh_18(self):
+        with numpy.errstate(divide='raise'):
+            assert_allclose(
+                npf.nper(0.1, 0, -500, 1500),
+                11.52670461,  # Computed using Google Sheet's NPER
+            )
+
+    def test_infinite_payments(self):
+        with numpy.errstate(divide='raise'):
+            result = npf.nper(0, -0.0, 1000)
+        assert_(result == numpy.inf)
+
+    def test_no_interest(self):
+        assert_(npf.nper(0, -100, 1000) == 10)