Add weekday method to date object

src/undate/date.py

@@ -104,6 +104,27 @@ def day(self) -> Optional[int]:
             return int(str(self.astype("datetime64[D]")).split("-")[-1])
         return None
 
+    @property
+    def weekday(self) -> Optional[int]:
+        """Equivalent to :meth:`datetime.date.weedkay`; returns day of week as an
+        integer where Monday is 0 and Sunday is 6. Only supported for dates
+        with date unit in days.
+        """
+        # only return a weekday if date unit is in days
+        if self.dtype == "datetime64[D]":
+            # calculate based on difference between current day and week start
+            # numpy datetime weeks start on thursdays - presumably since
+            # unix epoch day zero was a thursday...
+
+            # implementation inspired in part by https://stackoverflow.com/a/54264187
+
+            thursday_week = self.astype("datetime64[W]")
+            days_from_thursday = (self - thursday_week).astype(int)
+            # if monday is 0, thursday is 3
+            return (days_from_thursday + 3) % 7
+
+        return None
+
     def __sub__(self, other):
         # modify to conditionally return a timedelta object instead of a
         # Date object with dtype timedelta64[D] (default behavior)

tests/test_date.py

@@ -1,3 +1,5 @@
+import datetime
+
 import numpy as np
 from undate.date import ONE_YEAR, Date, DatePrecision, Timedelta
 
@@ -51,6 +53,26 @@ def test_properties_day(self):
         assert Date(2010, 5).day is None
         assert Date(2021, 6, 15).day == 15
 
+    def test_weekday(self):
+        # thursday
+        assert Date(2025, 1, 2).weekday == 3
+        assert Date(2025, 1, 2).weekday == datetime.date(2025, 1, 2).weekday()
+        # friday
+        assert Date(2025, 1, 3).weekday == 4
+        assert Date(2025, 1, 3).weekday == datetime.date(2025, 1, 3).weekday()
+        # saturday
+        assert Date(2025, 1, 4).weekday == 5
+        assert Date(2025, 1, 4).weekday == datetime.date(2025, 1, 4).weekday()
+        # sunday
+        assert Date(2025, 1, 5).weekday == 6
+        assert Date(2025, 1, 5).weekday == datetime.date(2025, 1, 5).weekday()
+        # monday
+        assert Date(2025, 1, 6).weekday == 0
+        assert Date(2025, 1, 6).weekday == datetime.date(2025, 1, 6).weekday()
+        # tuesday
+        assert Date(2025, 1, 7).weekday == 1
+        assert Date(2025, 1, 7).weekday == datetime.date(2025, 1, 7).weekday()
+
     def test_substract(self):
         # date - date = timedelta
         date_difference = Date(2024, 1, 2) - Date(2024, 1, 1)