scaling.wvm: return unchanged signal for a single site

docs/sphinx/source/whatsnew/v0.16.2.rst

@@ -16,6 +16,8 @@ Bug fixes
 ~~~~~~~~~
 * :py:func:`pvlib.tracking.calc_surface_orientation` returns scalar when input
   is scalar (:issue:`2881`, :pull:`2882`)
+* :py:func:`pvlib.scaling.wvm` no longer returns all NaNs when ``positions``
+  contains a single site (:issue:`2895`, :pull:`2896`) by :ghuser:`bfs2021`
 
 Enhancements
 ~~~~~~~~~~~~

pvlib/scaling.py

@@ -127,6 +127,11 @@ def _compute_vr(positions, cloud_speed, tmscales):
     # Added by Joe Ranalli (@jranalli), Penn State Hazleton, 2021
 
     pos = np.array(positions)
+    if len(pos) <= 1:
+        # A single site has no pairwise distances, so the variability
+        # reduction is 1; the effective-size optimizer below would converge
+        # to n_dist=0 and divide by zero. See GH #2895.
+        return np.ones(tmscales.shape)
     dist = pdist(pos, 'euclidean')
 
     # Find effective length of position vector, 'dist' is full pairwise

tests/test_scaling.py

@@ -207,3 +207,17 @@ def test_wvm_series_xyaslist(clear_sky_index, time, positions,
 def test_wvm_invalid(clear_sky_index, positions):
     with pytest.raises(ValueError):
         scaling.wvm(clear_sky_index, positions, cloud_speed)
+
+
+def test_compute_vr_single_site(expect_tmscale):
+    vr = scaling._compute_vr(np.array([(0., 0.)]), cloud_speed,
+                             np.array(expect_tmscale))
+    assert_almost_equal(vr, np.ones(len(expect_tmscale)))
+
+
+def test_wvm_single_site(clear_sky_index):
+    # A single site has no variability reduction: the smoothed signal is the
+    # input unchanged. Previously returned all NaNs. See GH #2895.
+    single_site = np.array([(0., 0.)])
+    cs_sm, _, _ = scaling.wvm(clear_sky_index, single_site, cloud_speed, dt=dt)
+    assert_almost_equal(cs_sm[5000:5005], clear_sky_index[5000:5005])