Correct slope calculation in clearsky.detect_clearsky (#1242)

* correct slope calculation, add test with time interval 2 minutes

* remove unneeded test adjustment

* add note to whatsnew

* avoid diff twice

Author: cwhanse

docs/sphinx/source/whatsnew/v0.9.0.rst

@@ -172,6 +172,9 @@ Bug fixes
   calling :py:func:`~pvlib.pvsystem.calcparams_cec`. (:issue:`1215`, :pull:`1216`)
 * Corrected methodology error in :py:func:`~pvlib.scaling.wvm`. Tracks with
   fix in PVLib for MATLAB. (:issue:`1206`, :pull:`1213`)
+* Corrected an error affecting :py:func:`~pvlib.clearsky.detect_clearsky`
+  when data time step is not one minute. Error was introduced in v0.8.1.
+  (:issue:`1241`, :pull:`1242`)
 
 Testing
 ~~~~~~~

pvlib/clearsky.py

@@ -652,17 +652,18 @@ def _calc_stats(data, samples_per_window, sample_interval, H):
     # shift to get forward difference, .diff() is backward difference instead
     data_diff = data.diff().shift(-1)
     data_slope = data_diff / sample_interval
-    data_slope_nstd = _slope_nstd_windowed(data, H, samples_per_window)
+    data_slope_nstd = _slope_nstd_windowed(data_slope.values[:-1], data, H,
+                                           samples_per_window, sample_interval)
     data_slope_nstd = data_slope_nstd
 
     return data_mean, data_max, data_slope_nstd, data_slope
 
 
-def _slope_nstd_windowed(data, H, samples_per_window):
+def _slope_nstd_windowed(slopes, data, H, samples_per_window, sample_interval):
     with np.errstate(divide='ignore', invalid='ignore'):
-        raw = np.diff(data)
-        raw = raw[H[:-1, ]].std(ddof=1, axis=0) / data.values[H].mean(axis=0)
-    return _to_centered_series(raw, data.index, samples_per_window)
+        nstd = slopes[H[:-1, ]].std(ddof=1, axis=0) \
+            / data.values[H].mean(axis=0)
+    return _to_centered_series(nstd, data.index, samples_per_window)
 
 
 def _max_diff_windowed(data, H, samples_per_window):

pvlib/tests/test_clearsky.py

@@ -607,6 +607,17 @@ def test_detect_clearsky_window(detect_clearsky_data):
                         check_dtype=False, check_names=False)
 
 
+def test_detect_clearsky_time_interval(detect_clearsky_data):
+    expected, cs = detect_clearsky_data
+    u = np.arange(0, len(cs), 2)
+    cs2 = cs.iloc[u]
+    expected2 = expected.iloc[u]
+    clear_samples = clearsky.detect_clearsky(
+        expected2['GHI'], cs2['ghi'], window_length=6)
+    assert_series_equal(expected2['Clear or not'], clear_samples,
+                        check_dtype=False, check_names=False)
+
+
 def test_detect_clearsky_arrays(detect_clearsky_data):
     expected, cs = detect_clearsky_data
     clear_samples = clearsky.detect_clearsky(