first working version of a charts/power view;

flexmeasures/api/common/schemas.py

@@ -4,6 +4,7 @@
 from marshmallow import fields, ValidationError
 import isodate
 from isodate.isoerror import ISO8601Error
+import pandas as pd
 
 
 class DurationField(fields.Str):
@@ -43,8 +44,15 @@ def ground_from(
         converting to a datetime.timedelta is not possible (no obvious
         number of days). In this case, `_deserialize` returned an
         `isodate.Duration`. We can derive the timedelta by grounding to an
-        actual time span, for which we require a start datetime.
+        actual time span, for which we require a timezone-aware start datetime.
         """
         if isinstance(duration, isodate.Duration) and start:
-            return (start + duration) - start
+            years = duration.years
+            months = duration.months
+            days = duration.days
+            seconds = duration.tdelta.seconds
+            offset = pd.DateOffset(
+                years=years, months=months, days=days, seconds=seconds
+            )
+            return (pd.Timestamp(start) + offset).to_pydatetime() - start
         return duration

flexmeasures/api/common/tests/test_schemas.py

@@ -1,6 +1,7 @@
 from datetime import datetime, timedelta
 
 import pytest
+import pytz
 import isodate
 from marshmallow import ValidationError
 
@@ -30,6 +31,12 @@ def test_duration_field_straightforward(duration_input, exp_deserialization):
         ("P1M", isodate.Duration(months=1), timedelta(days=29)),
         ("PT24H", isodate.Duration(hours=24), timedelta(hours=24)),
         ("P2D", isodate.Duration(hours=48), timedelta(hours=48)),
+        # following are calendar periods including a transition to daylight saving time (DST)
+        ("P2M", isodate.Duration(months=2), timedelta(days=60) - timedelta(hours=1)),
+        # ("P8W", isodate.Duration(days=7*8), timedelta(weeks=8) - timedelta(hours=1)),
+        # ("P100D", isodate.Duration(days=100), timedelta(days=100) - timedelta(hours=1)),
+        # following is a calendar period with transitions to DST and back again
+        ("P1Y", isodate.Duration(years=1), timedelta(days=366)),
     ],
 )
 def test_duration_field_nominal_grounded(
@@ -38,12 +45,16 @@ def test_duration_field_nominal_grounded(
     """Nominal durations are tricky:
     https://en.wikipedia.org/wiki/Talk:ISO_8601/Archive_2#Definition_of_Duration_is_incorrect
     We want to test if we can ground them as expected.
-    We use a particular datetime to ground, in a leap year february.
+    We use a particular datetime to ground, in a leap year February.
+    For the Europe/Amsterdam timezone, daylight saving time started on March 29th 2020.
+    # todo: the commented out tests would work if isodate.parse_duration would have the option to stop coercing ISO 8601 days into datetime.timedelta days
     """
     df = DurationField()
     deser = df.deserialize(duration_input, None, None)
     assert deser == exp_deserialization
-    dummy_time = datetime(2020, 2, 22, 18, 7)
+    dummy_time = pytz.timezone("Europe/Amsterdam").localize(
+        datetime(2020, 2, 22, 18, 7)
+    )
     grounded = DurationField.ground_from(deser, dummy_time)
     assert grounded == grounded_timedelta