BUG: take the fv result dtype from all inputs, not just rate

numpy_financial/_financial.py

@@ -242,7 +242,9 @@ def fv(rate, nper, pmt, pv, when: _When = 'end'):
     when = _convert_when(when)
     rate, nper, pmt, pv, when = np.broadcast_arrays(rate, nper, pmt, pv, when)
 
-    fv_array = np.empty_like(rate)
+    # An integer `rate` on its own would give an integer buffer, truncating
+    # the result, so take the dtype from all the inputs (gh-166).
+    fv_array = np.empty_like(rate, dtype=np.result_type(rate, nper, pmt, pv, 1.0))
     zero = rate == 0
     nonzero = ~zero
 

numpy_financial/tests/test_financial.py

@@ -579,6 +579,11 @@ def test_broadcast_decimal(self, when, desired):
         result = npf.ppmt(*args) if when is None else npf.ppmt(*args, when)
         assert_decimal_close(result, desired, tol=1e-8)
 
+    def test_gh_166(self):
+        # An integer rate gave an integer buffer, so storing NaN in it
+        # raised a ValueError.
+        assert_allclose(npf.ppmt(0, 1, 12, 1200), -100.0, rtol=1e-10)
+
 
 class TestIpmt:
     def test_float(self):
@@ -679,6 +684,11 @@ def test_0d_inputs(self):
         # 0d array inputs should return a scalar.
         assert numpy.isscalar(npf.ipmt(*args))
 
+    def test_gh_166(self):
+        # An integer rate gave an integer buffer, so storing NaN in it
+        # raised a ValueError.
+        assert_allclose(npf.ipmt(0, 1, 12, 1200), 0.0, rtol=1e-10)
+
 
 class TestFv:
     def test_float(self):
@@ -749,6 +759,17 @@ def test_some_rates_zero(self):
             rtol=1e-10,
         )
 
+    @pytest.mark.parametrize(
+        "args, desired",
+        [
+            ((0, 12, -100.5, -1000.25), 2206.25),
+            (([0, 0], 12, -100.5, -1000.25), [2206.25, 2206.25]),
+        ],
+    )
+    def test_gh_166(self, args, desired):
+        # An integer rate gave an integer buffer, truncating the result.
+        assert_allclose(npf.fv(*args), desired, rtol=1e-10)
+
 
 class TestIrr:
     def test_npv_irr_congruence(self):