Add error check to detect_clearsky

docs/sphinx/source/whatsnew/v0.11.2.rst

@@ -25,6 +25,8 @@ Documentation
   page. (:issue:`2202`, :pull:`2226`)
 * Updated :py:func:`~pvlib.irradiance.reindl` to include definitions of terms
   and a new "notes" section (:issue:`2183`, :pull:`2193`)
+* Clarified the error message in :py:func:`~pvlib.clearsky.detect_clearsky` when
+  windows contain fewer than three data points (:issue:`2005`, :pull:`2281`)
 
 Testing
 ~~~~~~~

pvlib/clearsky.py

@@ -713,8 +713,8 @@ def detect_clearsky(measured, clearsky, times=None, infer_limits=False,
         If True, does not use passed in kwargs (or defaults), but instead
         interpolates these values from Table 1 in [2]_.
     window_length : int, default 10
-        Length of sliding time window in minutes. Must be greater than 2
-        periods.
+        Length of sliding time window in minutes. Each window must contain at
+        least three data points.
     mean_diff : float, default 75
         Threshold value for agreement between mean values of measured
         and clearsky in each interval, see Eq. 6 in [1]. [W/m2]
@@ -758,9 +758,13 @@ def detect_clearsky(measured, clearsky, times=None, infer_limits=False,
     Raises
     ------
     ValueError
-        If measured is not a Series and times is not provided
+        If measured is not a Series and times is not provided.
+    ValueError
+        If a window contains less than three data points.
+    ValueError
+        If the measured data is not sufficient to fill a window.
     NotImplementedError
-        If timestamps are not equally spaced
+        If timestamps are not equally spaced.
 
     References
     ----------
@@ -812,6 +816,12 @@ def detect_clearsky(measured, clearsky, times=None, infer_limits=False,
     sample_interval, samples_per_window = \
         tools._get_sample_intervals(times, window_length)
 
+    if samples_per_window < 3:
+        raise ValueError(f"Window of length {window_length} found"
+                         " Each window must contain at least"
+                         " three data points; {samples_per_window} results"
+                         " from time interval of {sample_interval}")
+
     # if infer_limits, find threshold values using the sample interval
     if infer_limits:
         window_length, mean_diff, max_diff, lower_line_length, \

pvlib/tests/test_clearsky.py

@@ -674,10 +674,16 @@ def test_detect_clearsky_missing_index(detect_clearsky_data):
 
 def test_detect_clearsky_not_enough_data(detect_clearsky_data):
     expected, cs = detect_clearsky_data
-    with pytest.raises(ValueError, match='have at least'):
+    with pytest.raises(ValueError, match='times has only'):
         clearsky.detect_clearsky(expected['GHI'], cs['ghi'], window_length=60)
 
 
+def test_detect_clearsky_window_too_short(detect_clearsky_data):
+    expected, cs = detect_clearsky_data
+    with pytest.raises(ValueError, match="Window of length"):
+        clearsky.detect_clearsky(expected['GHI'], cs['ghi'], window_length=2)
+
+
 @pytest.mark.parametrize("window_length", [5, 10, 15, 20, 25])
 def test_detect_clearsky_optimizer_not_failed(
     detect_clearsky_data, window_length