NREL · mdeceglie · Nov 30, 2022 · Aug 1, 2022 · Aug 1, 2022 · Aug 2, 2022
diff --git a/docs/sphinx/source/changelog/pending.rst b/docs/sphinx/source/changelog/pending.rst
@@ -4,11 +4,12 @@ Pending
 
 Bug Fixes
 ---------
+* :py:func:`~rdtools.degradation.degradation_year_on_year` no longer raises
+  an error for inputs exactly two years long (:pull:`339`)
 * :py:func:`~rdtools.plotting.soiling_interval_plot` no longer ignores the optional
   ``point_color``, ``profile_color``, ``point_alpha``, and ``profile_alpha`` parameters.
   (:issue:`343`, :pull:`345`)
 
-
 Testing
 -------
 * Added a CI notebook check (:pull:`270`)
@@ -25,3 +26,10 @@ Requirements
 * Bump ``sphinx`` version from 3.2 to 4.5 and ``nbsphinx`` version
   from 0.8.5 to 0.8.8 in the optional ``[doc]`` requirements (:pull:`317`, :pull:`325`)
 * A number of other requirements updates (:pull:`337`)
+
+Contributors
+------------
+* Sandra Villamar (:ghuser:`SandraVillamar`)
+* Michael Deceglie (:ghuser:`mdeceglie`)
+* Chris Deline (:ghuser:`cdeline`)
+* Kevin Anderson (:ghuser:`kanderso-nrel`)
diff --git a/rdtools/degradation.py b/rdtools/degradation.py
@@ -231,11 +231,19 @@ def degradation_year_on_year(energy_normalized, recenter=True,
   
8000
      raise ValueError('energy_normalized must not be '
                          'more frequent than daily')
 
-    # Detect less than 2 years of data
-    if energy_normalized.index[-1] - energy_normalized.index[0] < \
-            pd.Timedelta('730d'):
-        raise ValueError('must provide at least two years of '
-                         'normalized energy')
+    # Detect less than 2 years of data. This is complicated by two things:
+    #   - leap days muddle the precise meaning of "two years of data".
+    #   - can't just check the number of days between the first and last
+    #     index values, since non-daily (e.g. weekly) inputs span
+    #     a longer period than their index values directly indicate.
+    # See the unit tests for several motivating cases.
+    if energy_normalized.index.inferred_freq is not None:
+        step = pd.tseries.frequencies.to_offset(energy_normalized.index.inferred_freq)
+    else:
+        step = energy_normalized.index.to_series().diff().median()
+
+    if energy_normalized.index[-1] < energy_normalized.index[0] + pd.DateOffset(years=2) - step:
+        raise ValueError('must provide at least two years of normalized energy')
 
     # Auto center
     if recenter:
@@ -263,6 +271,7 @@ def degradation_year_on_year(energy_normalized, recenter=True,
     df.index = df.dt
 
     yoy_result = df.yoy.dropna()
+
     df_right = df.set_index(df.dt_right).drop_duplicates('dt_right')
     df['usage_of_points'] = df.yoy.notnull().astype(int).add(
                 df_right.yoy.notnull().astype(int), fill_value=0)

diff --git a/rdtools/test/degradation_test.py b/rdtools/test/degradation_test.py
@@ -1,6 +1,7 @@
 """ Degradation Module Tests. """
 
 import unittest
+import pytest
 import sys
 
 import pandas as pd
@@ -164,6 +165,32 @@ def test_usage_of_points(self):
         self.assertTrue((np.sum(rd_result[2]['usage_of_points'])) == 1462)
 
 
+@pytest.mark.parametrize('start,end,freq', [
+    ('2014-01-01', '2015-12-31', 'D'),   # no leap day
+    ('2015-01-01', '2016-12-31', 'D'),   # leap day included in index
+    ('2015-01-01', '2016-12-29', '7D'),  # leap day in period but not in index
+    ('2016-06-01', '2018-05-31', 'D'),   # leap year, but no leap day in period
+    #  ('2016-02-29', '2018-02-28', 'd'),   # starts on leap day (doesn't work)
+    ('2014-03-01', '2016-02-29', 'D'),   # ends on leap day
+    ('2015-01-01', '2016-12-31', 'M'),   # month end
+    ('2015-01-01', '2016-12-31', 'MS'),   # month start
+])
+def test_yoy_two_years_error(start, end, freq):
+    # GH 339
+    times = pd.date_range(start, end, freq=freq)
+    series = pd.Series(1, index=times)
+    # introduce NaN at the end to ensure that the 2 year requirement applies to
+    # timestamps, not non-nan values:
+    series.iloc[-5:] = np.nan
+    # should not raise an error
+    _ = degradation_year_on_year(series)
+    # but if we shorten it by one element, then it should:
+    with pytest.raises(ValueError, match='must provide at least two years'):
+        _ = degradation_year_on_year(series.iloc[:-1])
+    with pytest.raises(ValueError, match='must provide at least two years'):
+        _ = degradation_year_on_year(series.iloc[1:])
+
+
 if __name__ == '__main__':
     # Initialize logger when run as a module:
     #     python -m tests.degradation_test