DRAFT: Handle dni/dhi=0/0 in irradiance.perez without NaNs

pvlib/irradiance.py

@@ -1147,12 +1147,21 @@ def perez(surface_tilt, surface_azimuth, dhi, dni, dni_extra,
     kappa = 1.041  # for solar_zenith in radians
     z = np.radians(solar_zenith)  # convert to radians
 
-    # delta is the sky's "brightness"
+    # delta is the sky's "brightness", assumes dni_extra > 0
     delta = dhi * airmass / dni_extra
 
-    # epsilon is the sky's "clearness"
-    with np.errstate(invalid='ignore'):
-        eps = ((dhi + dni) / dhi + kappa * (z ** 3)) / (1 + kappa * (z ** 3))
+    # epsilon is the sky's "clearness". Preserves NaNs for dni or dhi.
+    # Assumes:
+    # - dni >=0 and dhi >= 0.
+    # - dni->0^+ faster than dhi->0^+.
+    irr_ratio = np.zeros(np.broadcast(dni, dhi).shape)
+    with np.errstate(divide="raise"):
+        irr_ratio = np.divide(
+            dni, dhi, out=irr_ratio, where=np.logical_not(
+                np.logical_and(dhi == 0, dni == 0)
+            )
+        )
+    eps = 1 + irr_ratio / (1 + kappa * (z ** 3))
 
     # numpy indexing below will not work with a Series
     if isinstance(eps, pd.Series):

tests/test_irradiance.py

@@ -7,8 +7,9 @@
 import pandas as pd
 
 import pytest
-from numpy.testing import (assert_almost_equal,
-                           assert_allclose)
+from numpy.testing import (
+    assert_almost_equal, assert_allclose, assert_equal, assert_raises
+)
 from pvlib import irradiance, albedo
 
 from .conftest import (
@@ -393,6 +394,17 @@ def test_perez_negative_horizon():
     assert_series_equal(sum_components, expected_for_sum, check_less_precise=2)
 
 
+def test_perez_zero_dhi_and_dni(dni_et):
+    out = irradiance.perez(20, 180, 0.0, 0.0, dni_et, 89.96, 256.28, 37.32)
+    expected = 0.0
+    assert_equal(out, expected)
+
+
+def test_perez_zero_dhi_nonzero_dni(dni_et):
+    with assert_raises(FloatingPointError):
+        irradiance.perez(20, 180, 0.0, 100.0, dni_et, 89.96, 256.28, 37.32)
+
+
 def test_perez_arrays(irrad_data, ephem_data, dni_et, relative_airmass):
     dni = irrad_data['dni'].copy()
     dni.iloc[2] = np.nan