Handle time normalization for nonexistent and ambiguous times

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

@@ -17,6 +17,8 @@ Bug Fixes
 ~~~~~~~~~
 * :py:meth:`~pvlib.pvsystem.PVSystem.get_irradiance` accepts float inputs.
   (:issue:`1338`, :pull:`2227`)
+* Handle DST transitions that happen at midnight in :py:func:`pvlib.solarposition.hour_angle`
+  (:issue:`2132` :pull:`2133`)
 
 Bug fixes
 ~~~~~~~~~
@@ -74,3 +76,4 @@ Contributors
 * matsuobasho (:ghuser:`matsuobasho`)
 * Echedey Luis (:ghuser:`echedey-ls`)
 * Kevin Anderson (:ghuser:`kandersolar`)
+* Scott Nelson (:ghuser:`scttnlsn`)

pvlib/solarposition.py

@@ -1349,6 +1349,12 @@ def hour_angle(times, longitude, equation_of_time):
     times : :class:`pandas.DatetimeIndex`
         Corresponding timestamps, must be localized to the timezone for the
         ``longitude``.
+
+        A `pytz.exceptions.AmbiguousTimeError` will be raised if any of the
+        given times are on a day when the local daylight savings transition
+        happens at midnight.  If you're working with such a timezone,
+        consider converting to a non-DST timezone (e.g. GMT-4) before
+        calling this function.
     longitude : numeric
         Longitude in degrees
     equation_of_time : numeric
@@ -1421,7 +1427,17 @@ def _times_to_hours_after_local_midnight(times):
     if not times.tz:
         raise ValueError('times must be localized')
 
-    hrs = (times - times.normalize()) / pd.Timedelta('1h')
+    # Some timezones have a DST shift at midnight:
+    #   11:59pm -> 1:00am - results in a nonexistent midnight
+    #   12:59am -> 12:00am - results in an ambiguous midnight
+    # We remove the timezone before normalizing for this reason.
+    naive_normalized_times = times.tz_localize(None).normalize()
+
+    # Use Pandas functionality for shifting nonexistent times forward
+    normalized_times = naive_normalized_times.tz_localize(
+        times.tz, nonexistent='shift_forward', ambiguous='raise')
+
+    hrs = (times - normalized_times) / pd.Timedelta('1h')
 
     # ensure array return instead of a version-dependent pandas <T>Index
     return np.array(hrs)

pvlib/tests/test_solarposition.py

@@ -8,6 +8,7 @@
 from .conftest import assert_frame_equal, assert_series_equal
 from numpy.testing import assert_allclose
 import pytest
+import pytz
 
 from pvlib.location import Location
 from pvlib import solarposition, spa
@@ -711,6 +712,38 @@ def test_hour_angle():
         solarposition._local_times_from_hours_since_midnight(times, hours)
 
 
+def test_hour_angle_with_tricky_timezones():
+    # GH 2132
+    # tests timezones that have a DST shift at midnight
+
+    eot = np.array([-3.935172, -4.117227, -4.026295, -4.026295])
+
+    longitude = 70.6693
+    times = pd.DatetimeIndex([
+        '2014-09-06 23:00:00',
+        '2014-09-07 00:00:00',
+        '2014-09-07 01:00:00',
+        '2014-09-07 02:00:00',
+    ]).tz_localize('America/Santiago', nonexistent='shift_forward')
+
+    with pytest.raises(pytz.exceptions.NonExistentTimeError):
+        times.normalize()
+
+    # should not raise `pytz.exceptions.NonExistentTimeError`
+    solarposition.hour_angle(times, longitude, eot)
+
+    longitude = 82.3666
+    times = pd.DatetimeIndex([
+        '2014-11-01 23:00:00',
+        '2014-11-02 00:00:00',
+        '2014-11-02 01:00:00',
+        '2014-11-02 02:00:00',
+    ]).tz_localize('America/Havana', ambiguous=[True, True, False, False])
+
+    with pytest.raises(pytz.exceptions.AmbiguousTimeError):
+        solarposition.hour_angle(times, longitude, eot)
+
+
 def test_sun_rise_set_transit_geometric(expected_rise_set_spa, golden_mst):
     """Test geometric calculations for sunrise, sunset, and transit times"""
     times = expected_rise_set_spa.index